Infinity on Fire. The Dimensional Alliance 2nd edition, #2

Jenny has the weight of the multiverse on her shoulders and the fate of everyone she cares about in her hands. Ever since her aunt, who she barely knew, left the strange little house on Infinity Loop to Jenny in her will, along with the cat, Tidbit, and a tiny key on a gold chain, everything had changed for her. Never would she have expected that an impossible door in her new house would lead her to adventures beyond her own universe with beings that seemed to come right out of the myths and legends of Earth and beyondNow she must organize the forces of the Dimensional Alliance to fight against a persistent and formidable enemy who would enslave the population of every inhabited planet in the multiverse, if they have their way, plunging the multiverse into never-ending warfare and strife.