I cannot think of any team that benefits from having a Blood Reaper on it. Seriously. If you’re just looking to proc an ally death, use Dark Ritual or Carver Sadist, both of which have a much better benefit. I can just barely eke out some synergy with Link - because the Barrier created is dependent on Max HP, not current HP, and this makes the Blood Reaper fairly survivable - but that doesn’t fix the core issue.
The problem is this: the Reaper sacrifices an ally to save its own life, but the ability of the creature you just saved is literally only blowing allies up to make itself not die. Any other ability on any other creature, no matter how lame or specific, is better than this, because at least they don’t give the enemy team a free shot past Defense/HP buffs, Shell/Mend/Protect/Invisibility, and Provoke/Taunt. Even if the other team chooses to ignore the Blood Reaper, it’s no big loss, because while it has a decent base 38 attack, that’s nothing you don’t find all the time on other creatures that bring actually useful things to the table all on their own.
I recommend one of two things: either buffing the Blood Reaper’s attack enough that keeping it alive becomes meaningful, and not a leading cause of brain tumors, or altering its ability to grant some kind of additional buff each time it consumes another creature. Say, a 20% boost in Attack/Defense/Speed, or something like that. It’s still weak compared to the ridiculous proc of, for example Wicked Carver, but at least then when it’s the last creature standing it wouldn’t die like a pansy.