Originally Posted by
DragonsLover
Here's what I did: I turned the Horned Reapers and the little room they're in as white, including the door (otherwise, once you'd get the portal, they'd chop chop the Dungeon Heart). I deleted the Prison, but don't worry: once the Horned Reapers are dead, you get it through the room's panel with a "tutorial flash button" to tell you it has been added. That way, the player cannot imprison them. The only remaining thing would be to write down a message in the game to explain all that, something like "By killing the Horned Reapers, you discover on one of their corpses the plan to build a Prison!".
Sounds good?
Btw, Horned Reaper in North-West lava lake remains neutral.