you could transform into a very powerful beast form, but only for limited turns. After that you had to fight battles without using to be able to use it again safely..
That is a silly idea imo. Just a long cooldown in between fights, Baldur's Gate 2 had a reputation cost and you take damage in slayer form.
My idea would be the following :
Slayer is incredible powerful. Much more powerful than a fighter at lv cap. However, each turn in Slayer form sets your CON to 3 until you(not party members) kill a humanoid creature for each turn stayed in slayer form. That would fit the "lord of murder" power and if you use too often, you will only be able to fight with your dark gift.
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About tadpoles being just free power, I strongly agree. You lose nothing for becoming a flying tentacle psionic monstrosity.
Would be cool if there was many ways to remove the parasite, each one with good and bad effects. For eg, dying, leaving the parasyte to die and becoming undead, most notabely a vampire, it could be archived via bargaining with Cazador. Or making a homunclus clone and transferring your soul to the construct. It could be done bargaining with the owner of Sorcerous Sundries and offering to him what he really wants.
