How have you liked mistfire in general/how have people been playing it? I really like the card, but I believe your format is higher power.
I like the idea of it being a higher power mistfire analogue.
It's been a tad low power for my environment. The "problem" with the card is that it's a measly Hill Giant if you don't have any spells. If you can fire of multiple spells, it can totally break a stalemate by
jumping your entire team, which happens once in a blue moon, but most of the time it's used as a 4/4 flyer by casting a single spell. It also doesn't help that the ability is very proactive, i.e. you want to cast your spells before attacking. This clashes with the various counterspells, for more than the obvious reason actually, because Mistfire asks that you tap mana on your own turn to be able to swing in, meaning you have less (or no) mana on your opponent's turn for your counterspells.
I like Niblis of Frost, because it fills many of the same roles as Mistfire Adept. The prowess trigger is obvious, both turn into a 4/4 flyer, but like Mistfire Adept it allows you to break a stalemate by tapping down multiple problematic blockers. Unlike Mistfire Adept, it does something if you have no spells (it's still a 3/3 flyer), works well with counterspells, and plays
much better with instants, because you can tap down a creature for two turns.
Re:
Dungeon Geists; I've always been happy with the card while it was in my list, and it performs well, but I've axed it some time ago actually, because it's also pretty boring and generically good as an etb creature. I recently introduced
Icefall Regent to my cube. It's a mana more expensive, which I don't mind, etb effects are nice when they cost you something, and it's a more interesting card. The clock is a bit faster, and tacking
onto the cost of a removal spell means it trades in a mana neutral way a lot of the time. All in all Regent is both more fair and more interesting imo than Geists. In my list Regent also has the small benefit of working well with
Crux of Fate in the
control deck
Edit: Re: Tree of Perdition.
Obviously Emrakul changed our old "mythic" friend for the "worse"...
Quick! Someone make a combo deck in Standard!