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.

This build tops out at a converted mana cost of three, not counting the potential open-ended nature of the Titan’s Revenge. The goal here is to just keep dropping creatures and burn away any barriers (or just burn out the opponent). Maralen is a “late game” play for this deck, where her triple role is docking the opponent three per turn, attacking for two per turn, and tutoring up the most efficient burn for the situation. You absolutely do not play Maralen out in the face of a deck that can tutor up and play mass removal, as you don’t want to generate a many-for-one situation. Similarly, Maralen is not played out in the face of a combo deck that likely needs just one more piece to go off.
I’ve tested Maralen Sligh against Rock, and it works shockingly well (shockingly because that Rock build has taken apart every other aggro deck it’s faced). When I have time — perhaps after next week’s PTQ — I may bring this along to one of the local groups and see how it fares against a broader range of decks.