Nah, umbra is not a necessary card. It's occasionally a gamewinner, but is a dead card when you can't cheat out Doomguards or do not have cubes in hand. I have Umbra, but I don't use it in cubelock...
Umbra is strong against slow decks, but sucks against aggro. So, yeah, she can be replaced. Your complete list would help with a replacement, but in general Tar Creeper, Stonehill, Taldaram, Gnomeferatu come to mind
Taldaram is nice but i can't craft it bro, i don't have the dust. I can focus on getting 1 Faceless Manipulator, only costs 400 dust
This is the deck i use:
I ain't worried about the meta, i play what i enjoy.
I've checked out so many cube decks and all run Spiritsinger Umbra,, are there any alternative cards i can use instead of it?.