Reyes was fortunate enough to hear the incantation, and also to hear it alone, with no one else around. She wasn't high up on the metaphorical totem pole, and likely would have been pushed aside if anyone else had heard. Even if the incantation was pronounced horribly (which it was, to her refined hearing), a disgrace to its power, any creature of the Underworld would have eagerly snatched up the chance to see who was summoning one of them. But there was no one else besides her.
And so she followed the call, choosing to act of her own accord before it dragged her along with it. The transition was quick and clean enough for her, but for the waiting Spellpaw, well, perhaps books were falling down all around him, perhaps the library shelves were shaking. Summoning a demon always had some effect on the mortal world. Reyes narrowed her eyes, preparing for when she moved out of the Underworld. Maybe she could have a bit of fun with whoever this person was. It was a change from being in the dark realms for all eternity., at least.
She first appeared as a smoky mist in front of Spellpaw, nothing but blackness and a pair of glowing red eyes. She coalesced quickly, though, collapsing down into a form of a black deer. Wings fluttered open at her sides, antlers adding a striking effect to her appearance. Her red eyes had faded into a light silver, so pale that they looked almost colorless. Barely a moment passed by before she'd lunged forward, spreading her wings wide for extra effect. "Who dares summon me?" she thundered, striking one hoof against the floor. "Who are you, mortal?"