The Headbutt doesn't do a whole lot of damage, or even appear to rattle Chimera very much...but at least Assumption's bony head will probably keep him from suffering from having headbutted basically a large rock.

...but, strangely, Chimera does let go of the bone and let Assumption yank it back, although it's more because:

"Whirlpool!"

Over the years of Pokemon battling, Envy's gotten mostly used to say, the sight of a water vortex spiraling up out of literally nowhere while there's also a sandstorm. But it's still weird as hell to actually see.