The Creeping Shadow
The Creeping Shadow was a magical shadow-creature controlled by the Draconic Sorceress in one universe. It had some degree of mass, although ordinary weapons couldn't harm it, and behaved somewhat like a huge snake. It had the ability to destroy everything it touched and a degree of intelligence.
Gwenivar, Wanderer in Everywhere was fighting the Creeping Shadow when a very different world's own incarnation of Jenny Everywhere, who was just starting to learn how to control her shifting room, accidentally summoned it into her university dorm room room. It began to rampage across the Neptunian Orbital University. Jenny managed to then summon Gwenivar herself, who gave her and Laura Drake enough information about the creature's nature that they were able to destroy it before it started eating its way through the other dorms. Using a rebuilt version of Laura's malfunctioning photon-freezer, they transformed it into a harmless mass of soap bubbles. Gwenivar then touched it when she shifted back home, taking it back to her universe along with herself. (PROSE: Extradimensional Experiments)