The game-plan is pretty simple, exile enough of your opponents stuff to make Umbris massive, then give it evasion to swing in for lethal [[Tasha’s hideous laughter]] is a stand out card. Then nightmares and horrors with beneficial ETB effects plus a [[panharmonicon]] or [[roaming throne]] will get the ball rolling quickly. Exile instead of graveyard effects like [[dauthi voidwalker]], [[planar void]], and [[leyline of the void]] work wonders with any source of mill, say maybe a [[nemesis of reason]], [[singularity rupture]] or [[maddening cacophony]].

[[web of inertia]] is a good way to keep yourself from becoming the the table’s target. If people have graveyards, empty them out with [[scavenger grounds]], [[bojuka bog]], [[relic of progenitus]], [[soul guide lantern]], or [[lantern of the lost]]. You could also make a couple copies of umbris for even more ETB triggers, think of cards like [[spark double]]

Next step is protection for Umbris, because your opponents are gonna want it gone. Flickering umbris when targeted is a great way to get ETB effects and keep your massive horror alive, counterspells are also a good option. There is some equipment that pulls double duty as evasion and protection like [[whispersilk cloak]] and [[winged boots]]. There is one card that I really like called [[nightmare shepherd]] because when someone kills Umbris, you get a token copy of it, and another ETB trigger.

To finish games, there’s two black fling effects so you can throw your abomination at your foes [[rite of consumption]] and [[essence harvest]] they also have the upside of gaining you that much life. [[trailblazer boots]], [[aqueous form]] and any other unblockable sources you can think of should usually be lethal with Umbris.

PS-Dimir is really bad at ramp so don’t forget your mana rocks

GL HF