A Soul Slayer (Zanpakuto in Japanese and referred to here on out) is a sword that a Soul Reaper (Shinigami in Japanese and referred to as that here on out) yields. The sword's look reflects the user itself by the means of details like the hilt and cross guard. A Zanpakuto comes in 3 ways. The first is the sealed form, followed by two levels of seal release: the initial release (Shikai), and final release (Bankai). In the sealed form, most Zanpakuto resemble katanas, and do not exhibit any special abilities. The greater a shinigami's spiritual power, the larger the sword appears. As such, Isshin Kurosaki notes that high level shinigami must be capable of maintaining and suppressing the size of their sword, otherwise shinigami like the captains of the Gotei 13 would end up swinging around swords the size of skyscrapers.
A properly-trained shinigami can make his or her Zanpakuto smaller if they wish, or radically change its appearance (Kisuke Urahara's Benihime is, in sealed form, a cane). Shinigami usually carry their Zanpakuto in the sealed state, releasing them only when necessary, while there are shinigami, notably Ichigo Kurosaki has his Zanpakuto continuously in released form. Soul Society law prohibits the release of Zanpakuto within the walled city of Seireitei in times of peace. The literal meaning means "soul cutting sword". The art of using a Zanpakuto is called zanjutsu (swordsmanship).
Most Zanpakuto's in their sealed form look like katanas, but the shape and form varies by the person. Zanpakuto's are not just tools, but also living beings. They are a shinigami's partner in battle and while they can regenerate if broken, they cannot be replaced. As a shinigami gains power, the Zanpakuto can change form by the inner being manifesting itself to a greater degree in the outside world. The sword and the inner being are synonymous to each other. Zanpakuto's are used to fight hollows and other Shinigami as well as to liberate or help pluses ascend to Soul Society.
Initial release
The first level of seal release, shikai, is achieved by learning the sword's name. By speaking a command phrase to the sword (unique to each shinigami), the Zanpakuto will assume its released form. In released form, most Zanpakuto have a different physical appearance, as well as one or more special abilities, often reflecting a strength or the personality of the wielder. Achievement of the initial release is a mark of expert control of a Zanpakuto, and it appears to be a requirement for advancement within the shinigami ranks (most seated officers and presumably all lieutenants are capable of the initial release).
Final release
The second and final level of seal release is called bankai (also called full release). It is a rare capability usually limited to captain and sometimes vice captain-level shinigami. Those who attain this power are remembered throughout shinigami history.
To become capable of the final release, a shinigami must materialize the sword's personification and subjugate it. In the final form, the Zanpakuto assumes a third physical appearance, and can fully manifest itself in the outside world, increasing its power by five to ten times. All Zanpakuto in the final form possess some type of powerful ability. Most skilled users also have several special attacks while using the final release, which require them to say a special command, similar to the initial release. Maintaining full release is taxing for the wielder, and cannot be held for exceedingly long periods of time.
The final release (bankai) generally requires years of training to acquire, and another ten years to master. However, there have been two exceptions to this: Ichigo Kurosaki and Kisuke Urahara. They acquired their final releases within three days of training, using a method pioneered by Urahara. Normally, a shinigami must have the strength and ability to manifest the sword's spirit into the real world before he can even begin his final release training; Urahara's method involves forcibly calling the spirit of the Zanpakuto into the user's world through the use of a special doll. Thus, the years of training to acquire the knowledge and strength to begin the final release training can be bypassed.

Ashisogi Jizou (Beast Killing)
Owner - Mayuri Kurotsuchi
Command: "Tear off!"
Unreleased: Looks like a normal sword
Initial release: Ashisogi Jizou (Beast Killing)
* Ability: Paralyzes Kurotsuchi's opponents when it hits, but allows them to feel more pain than normal. If Kurotsuchi stabs himself with it, he turns into liquid and cannot be attacked (but cannot attack either). He will require a few days to return back to normal form.
Final release: Konjiki Ashisogijizou (Golden Beast Killing)
* Ability: Becomes a large slug-like creature with a grotesque infant's head, armed with blades in its stomach. It emits a toxic gaseous poison over a large radius that will kill whomever inhales it. The chest can also reveal a cache of spike-like weapons which this monster can charge at it's opponent with.
-----------------------------------------------------------------------------------
Benihime
Owner - Urahara Kisuke
Command - "Awake"
Unreleased - Takes the form of a cane
Initial release: Benihime (Crimson Princess)
*Ability: Allows Urahara to manipulate blood. Can fire energy blasts of considerable strength [9] and create a blood-mist shield to protect Urahara.
-----------------------------------------------------------------------------------
Fujikujaku (Wysteria Peacock)
Owner - Ayasegame Yumichika
Command - "Bloom"
Unreleased: Looks like a normal sword
Initial release: Fuji Kujaku (Wisteria Peacock)
* Ability: Transforms into a fan of four scythe-like blades. Yumichika pretends this is his sword's only ability. Its true power is that it can absorb an opponent's spiritual energy. This is a kidō (demon arts) type ability, so Yumichika does not reveal it except to his enemies, for fear of being despised for it.
-----------------------------------------------------------------------------------
Haineko (Ash Cat)
Owner: Rangiku Matsumoto
Command: "Growl"
Unreleased - Looks like a normal sword
Initial release: Haineko (Ash Cat)
* Ability: The blade will first turn itself into a cloud of ash. While its direct effects on opponents are unknown, Haineko can cause considerable destruction to material, such as turning pillars of stone into piles of rubble.
-----------------------------------------------------------------------------------
Hozukimaru (Ghost Lamp)
Owner: Madarame Ikkaku
Command - "Grow"
Unreleased: Takes the form of a regular swoard
Initial release: Hozukimaru (Ghost Lamp)
* Ability: It takes on the appearance of a spear which leads many to believe that it is just that - a spear. It can then become a three-part staff and thus surprise opponents.
Final release: Ryumon Hozukimaru (Dragon Crest Ghost Lamp)
* Ability: It becomes a trio of huge axe-like blades connected by a chain with dragon design on them which begins to fill with crimson dye. When the design is completely filled, the full destructive force of Ikkaku's final release surfaces. However, the force of unleashing so much power can leave his Zanpakuto's edge brittle
-----------------------------------------------------------------------------------
Hyourinmaru (Ice Ring)
Owner: Hitsugaya Toushiro
Command: "Sit in the frozen heavens"
Unreleased - Looks like a normal sword
Initial release: Hyourinmaru (Ice Ring)
* Ability: Enables Tōshirō to exercise control over the weather, specifically wind and ice. It is noted to be the most powerful of all ice-based swords. The sword creates a Chinese style dragon of ice from the overflow of reiatsu. The only physical change in the sword is the addition of a chain with a crescent moon on the end.
Final release: Daiguren Hyourinmaru (Ice Ring of the Great Frozen Hell)
* Ability: Summons individually mobile ice "flowers", and creates a dragon of ice around Hitsugaya's arm, the mouth of which holds the handle of the sword (the fact it is a dragon is hard to notice until seen from a side view), and clawed, dragon-like feet of ice around Hitsugaya's own feet. The dragon also has wings of ice and a long tail he can use to hit and freeze an enemy with. It encompasses the battlefield with ice, controllable by Hitsugaya. Any water or ice that already exists there can also be controlled. The sword can be regenerated from atmospheric moisture if it is destroyed. Anyone cut by it is encased in a block of ice.
-----------------------------------------------------------------------------------
Katen Kyoukotsu (Flower Sky, Crazy Bones)
Owner: Kyouraku Shunsui
Command: Hana Kaze Midarete Kashin Naki Tenpū Midarete Tenma Warau (Flowers are disturbed, God of Flowers weeps, Winds of Heaven blow, Demon of Heaven laughs)
Unreleased - Looks like normal swords
Initial release: Katen Kyoukotsu (Flower Sky, Crazy Bones)
* Ability: Unknown, turns into two identical, broad, curved scimitars. There are only two Zanpakuto which are in a pair when in their released form.
-----------------------------------------------------------------------------------
Kyouka Suigetsu (Mirror Flower, Water Moon)
Owner: Aizen Sousuke
Command: "Shatter!"
Unreleased - Looks like a normal sword
Initial release: Kyouka Suigetsu (Mirror Flower, Water Moon)
* Ability: It is able to put anyone who sees it into a "complete hypnosis". Any time thereafter, Aizen can use this ability to create elaborate illusions that fool all five senses of anyone who has seen Kyouka Suigetsu's activation in the past. Due to the fact its release has to be seen, this power has no effect on the blind. Initially Aizen lied and said it has the ability to create a mist that could be used to create illusions in battle.
----------------------------------------------------------------------------
Ryujinjakka - (Flowing Blade like Flame)
Owner: Yamamoto Genryuusai Shigekuni
Command: "Disintegrate all existence to ashes!"
Unreleased - Takes the form of a cane
Initial release: Ryujinjakka - (Flowing Blade like Flame)
* Ability: Unknown, but the sheer power of its aura alone can disintegrate anything that Yamamoto waves the sword over into ash and engulf the area in flames. Even in its initial release, its power is strong enough for the owner to fight against two other captains simultaneously. This is the strongest of all fire-element swords. When sealed, Yamamoto's sword is disguised in the form of a heavy staff.
-----------------------------------------------------------------------------------
Senbonzakura (One Thousand Cherry Blossoms)
Owner: Kuchiki Byakuya
Command: "Scatter!"
Unreleased - Looks like a normal sword
Initial release: Senbonzakura (One Thousand Cherry Blossoms)
* Ability: The blade dissolves into a thousand razor-sharp blades, which swarm around the target and attack, appearing to be sakura petals in how the light reflects off of them. Their numbers and speed make them nearly impossible to parry.
Final release: Senbonzakura Kageyoshi (One Thousand Cherry Blossoms Brutal Shadows)
* Ability: Byakuya drops the blade into the ground and causes two rows of huge katana blades to rise from the ground and scatter into petals, forming a large scale version of the initial release ability. The petals can also compress into a large number a swords to strike the opponent (Senkei Senbonzakura Kageyoshi) or be arranged around Byakuya in a winglike form and allow him to perform one overwhelmingly powerful strike upon the opponent.
-----------------------------------------------------------------------------------
Shinsou (Divine Spear)
Owner: Ichimaru Gin
Command - "Shoot to kill"
Unreleased: Takes the form of a small sword
Initial release: Shinsou (Divine Spear)
* Ability: The blade elongates and shoots out at a bullet's speed, impaling its target even over a great distance. Also can launch a long beam that can inflict damage to enemies from far away.
-----------------------------------------------------------------------------------
Suzumebachi (Hornet)
Owner: Soi Fong
Command: "Sting all enemies to death!"
un release: Looks like a normal sword
Initial release: Suzumebachi (Hornet)
* Ability: Two hits in the same place leads to immediate death. The first hit marks Soi Fong's opponent with a butterfly signature called the hornet's crest; a second hit on the crest will cause instant death. Soi Fong has mastered this ability to such an extent that it will not fade unless she wills it.
-----------------------------------------------------------
Tobiume (Flying Plum)
Owner - Hinamori Momo
Command - "Snap"
Initial release: Tobiume (Flying ume tree)
* Ability: The blade straightens and produces several, Jitte-like prongs along its length. In this form, Tobiume can fire pink-colored energy bursts capable of destroying floors and holing walls or channel the energy into one fire ball
-----------------------------------------------------------------------------------
Wabisuke (Apology)
Owner - Kira Izuru
Command - "Raise your head"
Unreleased: Takes the form of a regular sword
Initial release: Wabisuke (Apology)
* Ability: The blade bends into a hook with two right angles. It can double the weight of anything it hits and generally causes an opponent's weapon to become too heavy to wield. And because the katana would be too heavy for the wielder to support, he will bend down, so that he bows his head, as if in apology - hence the name.
-----------------------------------------------------------------------------------
Zabimaru (Serpent Tail)
Owner - Abarai Renji
Command - "Howl"
Unreleased: Takes the form of a regular sword
Initial release: Zabimaru (Serpent Tail)
* Ability: The blade produces seven outward-jutting blades along its length and can be extended to great length and swung as a daggertail-like weapon. Each section is connected to the next by an elastic substance that allows the weapons elongation. It shouldn't be confused with his final release that is held together by his spiritual power. However, if Zabimaru is broken, Renji can use his spirtual power to levitate the individual segments of the blade to strike the opponent. The move is called Higa Zekkou, it hurts Zabimaru as this isn't how the weapon is supposed to be used. This is a one shot attack as Renji doesn't have the spiritual power to do it a second time.
Final release: Hihiō Zabimaru (Baboon King Zabimaru)
* Ability: The blade appears as a giant snake skeleton and can fully function as long as Renji has power left. All the sections of the skeleton are connected by his spiritual power, allowing him to connect and disconnect the sections of bone at will. That ability allows Zabimaru to continue fighting even if some sections are destroyed. Its attacks are a more powerful, larger-scale versions of Zabimaru's. It is however difficult to control its movement, due to its enormous size.
Spirit: Baboon-like creature with a snake for a tail
Other Information - This weapon attacks in a pattern of 3 phases. After the 3rd phase, Zabimaru is forced to contract leaving an opening.
-----------------------------------------------------------------------------------
Zangetsu (Moon Cutter)
Owner - Ichigo Kurosaki
Command - None, Ichigo's Zanpakuto is always in a released state
Un release: None, it is always released.
Initial release: Zangetsu (Moon Cutter)
* Ability - Is able to release and aim a large amount of spirit energy at an opponent. This attack is called the Zangeki also known as "Moon fang striking the sky".
Final release: Tensa Zangetsu (Heavenly Chain Cutting the Moon) - Changes the Zanpakuto's form to a long, thin, completely black sword. Concentrates spiritual energy and increases the user's speed drastically. In this stage Ichigo's clothes also change.
Spirit: An older man in a black cape or cloak with glasses. Ichigo calls him Old Man or Uncle. The hollow form of Ichigo also reveals itself to be the alter-ego of Zangetsu in a sense. It claims to be the dominant persona of Ichigo's Zanpakuto and Ichigo has subdued him, though the hollow forms promises to seize any chance to become the one that controls Ichigo's physical body.
Other Information - This Soul Slayer cannot be sealed due to the fact that it is too powerful to be sealed.