I like the mechanic of nightmares that refer to their exiled cards. This is one of the cleaner executions; Osmoting Arachnae has a cooler name, but much more fiddly text.

Yeah, this one was a bugger to get right. I initially wanted the Knight to choose one of the colors of the card exiled, but finding a way to say that simply proved too difficult. Eventually, I just figured it would be easier to let the players having something a little better.

Osmoting Arachnae is clunkier. If I come up with a better ability, I might just use that. Maybe a green drake that flys if you exile a flyer... I suppose it reads better that way. I do lose flavor points, though.