Under normal circumstances, it is prohibitively costly for a denizen of the Well to climb the steep energy gradient between it and the myriad universes above. But some factors lower the price that must be paid. Ripples in the Aether caused by multiversal travel, for example, create troughs of potential that can be exploited.
Full-scale invasions typically require inside help from within a universe. And so, when the reverberations from Ipaxalon's unexpected arrival on this Earth temporarily lower the costs of an incursion from prohibitive to merely exorbitant, a few more powerful denizens are willing to sponsor some clever and discreet minions to make the climb.
A particular sort of minion is quite well-suited to this task.