During your former travels with it, the pixie merely laughed at any suggestions that it give you its name. Based upon your books on summoning magic, that was probably wise of it; if you'd known its name you would have had to focus on it at all when summoning it.
The pixie's leaving quickly. With little time to think, you call after it. "Wait! Please, I'm sorry! I just wanted someone friendly to talk to, and you were so great- I'll make it up to you-"
The pixie's voice drifts to you from the darkness outside the keep, where you can see it floating in the dim moonlight. "None of that shit. We had a deal, you broke it, and you're not nearly as sorry as you will be."
You scowl. You were trying to be nice; why was it being so stubborn? And now it was threatening you... well, you could threaten back. "Fine!" You thrust your arm at it and gather your will, muttering well-learned syllables. A small bolt of energy flies at the pixie, but the quick little creature dodges and gets away clean.
"Hah! You'll have to do better than that, you incompetent- hrk-!" The pixie cuts off with a gurgle, clutching at its throat. You stare at it. What could have...
The assassin imp fades into view, its wings beating furiously. One of its arms is wrapped around the pixie's face, and the other holds a short blade driven deeply into the side of its neck. The pixie gurgles for a few long seconds more, then falls limp, and the assassin imp takes the body in both arms and carries at back towards the keep.
As it alights, it glances at you. "Well, that coulda gone better, eh? At least pixies are good eatin'!" It smiles, revealing a startling number of needle sharp teeth.
What do you do?