I am pretty sure this is the skull of a baby pterodactyl.
The jaw hinge point is on the bottom left and the beak break point is clearly visible on the left-most edge. Too bad that it is broken off. But super gnarly that the back-of-head-knife-attack-mode-spinity is totally intact.
Any foes I have should probably be worried. Friends need not worry though. I wouldn’t use its powers against friends.