[Snake]

Seems a bit uneven on what each effect does, and while I agree that they're literally in a state that anything they gain can help, why not go with this:

- Kouenjin
>> Hidden Cut: Deals bonus Fire Damage after the slash damage, equal to 100% of your Fire ATK, and automatically cast Explosion centered around the first enemy proc. (Sword will be the casting tool.)
>> On-Kill: All friendly Bombs and Explosions immediately trigger, and you gain Nerhaven LV4 for 4 rounds.

- Yukijin
>> Hidden Cut: Deals bonus Ice Damage after the slash damage, equal to 100% of your Ice ATK, and automatically cast Famiuga centered around the first enemy proc. (Sword will be the casting tool.)
>> On-Kill: Automatically use Expanding Ice for free, with your Sword's Power replacing a Fist weapon.

- Akujin
>> Hidden Cut: Deals bonus Dark Damage after the slash damage, equal to 100% of your Dark ATK, and automatically use Wretched Oil centered around the first enemy proc. (Sword will be the casting tool.)
>> On-Kill: All enemies within 4 range of the defeated enemy will be Feared for 4 rounds, and afflicted with Imagin's Despair.

- Sanjin
>> Hidden Cut: Deals bonus Acid Damage after the slash damage, equal to 100% of your Acid ATK, and automatically cast Acid Rain centered around the first enemy proc. (Sword will be the casting tool.)
>> On-Kill: All enemies within 4 range of the defeated enemy will take 25% of the defeated enemy's max HP as Acid magic damage.