I'm attacking the darkness, I cast magic missile.

Pooping is done out of combat, consider it like a weak summoning ritual. If you need it to be some sort of roll, a crit will give you +1 on your next roll, a crit fail though, means the summoning wasn't completely successful, and at some point, the rest of the summon will happen without your input.

is that an artificially yassified mark mercer

Nothing wrong with attuning to the barbarian.