Each room bringing new enemies, horrors, and surprises around each corner
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I imagine something like Entomb to tutor up a Bolas's Citadel, then casting Mox Amber to reanimate the Citadel and popping off from there could be a good start at what Moira can do, but I'm sure people can come up with far scarier lines than my quick example
Dropping him once you have an established board can get you very close to an instant ultimate, but dropping him early is still great, because he can make the way for your Ninjas to connect while giving you card advantage
If you have three or more lessons in your graveyard, you can attack and block with The Lion-Turtle, a strong 3/6 creature for 3 mana
Theyre already free value since they just happen, so making it happen multiple times in a turn is key to making the most of the mechanic