Its not especially fancy, but the mix of abilities is good enough to make it excellent on turn four - or a fine topdeck later on in the game, too
(Artifact, battle, creature, enchantment, instant, land, planeswalker, and sorcery are card types.) When Atraxa enters, reveal the top ten cards of your library
Just remember that umbra armor won't help if the creature's toughness is 0 or less, if it's being sacrificed, or if it's trying to leave the battlefield in any way other than by being destroyed
As an added boost to an already terrifying card, any time a ninja, turtle, and/or mutant deals combat damage to a player, that creature gets a +1/+1 counter