dnd 5th – New character Homebrew D & D

So I'm thinking of creating a new homebrew class built around an NPC. I almost want to do a fusion, the characters themselves are not writing anything, they may be good in a particular field, but when they are merged, they become really strong. I think that once they are below 25% of their power, they are forced to separate and are knocked out, each remaining half-power. I need decent ways to balance that if you can help me.