I have absolutely no idea what the thing in the background is supposed to be.
Can't recall any Pokémon with similar design and xurkitree which looks closest also desn't fit...

Behind Lusamine? That's Nihilego. Specifically, one that has fused with her.

Chiera said:

I have absolutely no idea what the thing in the background is supposed to be.
Can't recall any Pokémon with similar design and xurkitree which looks closest also desn't fit...

That's Nihilego.