Maralen Sligh
Maralen of the Mornsong is one of the standout "not as useful as you think it is" cards from Morningtide. I mentioned it twice, most recently in my review of interesting black cards from Morningtide. In general, people have been trying to find a place for Maralen in control and combo decks. I think this is a mistake, as letting your opponent tutor freely at the cost of damage is pretty much the opposite of a control strategy -- and the work involved in keeping them from tutoring is too much -- so much, in fact, that it dilutes the strength of your control or combo build.
As many have pointed out, the most common outcome for Maralen is that you'll play her, then your opponent will take three, tutor up removal, and kill her. There's exactly one place where this is perfectly fine -- a high-power aggro deck. Stick Maralen in a Rakdos/Sligh-style build, and she works out well. Decklist and a little more discussion in the extended.