Soul Reapers use a special weapon known as a Zanpakuto to extract the souls of slain foes and use their power again before sending them to the afterlife. The Zanpakuto starts off as a longsword that the Soul Reaper shrouds with part of their own soul to make it and extension of there own body. You gain the soul extraction ability, as a bonus action or reaction you can store the soul of a creature within 30 feet of you that  that died in the last 1 minute or less. You can store any number of souls and lose half your stored souls rounded down after a long rest.

Shikai

Your zanpakuto gains 2 abilities. The first ability it can take the form of any simple or martial weapon as a bonus action, gaining its mastery property. The modifier used by the weapon is strength and the damage does a D10 of the weapons damage or radiant damage even if the original weapon does more or less. You can expend a number of souls after you make a successful weapon attack to add an additional d10 of the same damage type for each soul used.

The second ability gives you access to 1st level cleric and wizard spells. You can prepare one spell from each list and can change the spells to a different one after a long rest. They use one soul per cast.

True Shinigami

Your zanpakuto has become more efficient at using the power of souls and sharpens itself from your own soul. Your soul is beginning to create its own pressure and you begin to feel like it's pushing back against the forces of gravity.

Your weapon damage now increases to a d12, as well as the damge from the souls when used after a hit. You can also expend a soul to roll a d12 after rolling to hit but before knowing the outcome and add it onto your roll.

You now gain access to level 2 wizard and cleric spells. You can choose 1 spell from each list to prepare after a long rest with your first level spells. They each cost one soul to cast.

You may now take the dash action as a bonus action.

Bankai

You have bonded with your zanpakuto long enough that it has gained its own soul and you learn it's true name. As a bonus action you say,  bankai[ zanpakuto true name] and you will enter your bankai release form.

You may use 3 souls to enter your bankai form for 1 minute. 

While in this form you gain +3 to all saves and +3 to hit with attacks and your movement speed increases by 20 feert  All your attacks do 2d12 damage and you may cast each of your prepared spells free one time each. No other souls may be spent while in bankai form but all souls of slain creatures that enter your radius are automatically extracted.

You gain access to level 3 wizard and cleric spells. You can prepare one of each after a long rest along with your other 4 spells. They cost one soul each to cast.

Gotei Captain

You have refined your skills and reached a level of control over souls at master level. You control your zanpakuto with amazing precision and can use fallen souls at an even higher potential. You can even move your own soul to transcend the normal movements of your body while on the battlefield.

While in bankai form, you gain a fly speed equal to your movement speed.

Your zanpakuto gains an affinity to one element of your choice either, acid, lightning, cold, fire, thunder or poison. All your attacks deal an additional d8 of the chosen elements damage.

You gain access to level 4 cleric and wizard spells list. You can prepare one of each along with your other 6 spells after a long rest. They cost one soul each to cast and can be cast for free one time each while in bankai form.

Soul King

You are the highest authority over souls. Your bond with your zanpakuto has reached its ultimate potential. You can hold onto souls as long as you need, no longer losing souls after a long rest.

While in Bankai Form your soul pressure hides your body making you almost completely transparent. All attacks against you and perception checks to see you have disadvantage. You can go into stealth with no cover and gain +10 to all stealth checks.

Your zanpakuto now does 2d8 of its elemental damage with each attack.

You gain access to level 5 cleric and wizard spells list. You can prepare 1 from each list along with your 8 other spells after a long rest. They cost 1 soul each to cast and can be cast for free 1 time each while in bankai form.