Technicalities
As Thor started to come of age, Amora recognized that it was only a matter of time before her “friend”, second in line to the throne, would make a grab for power, in which event she would do well to have her pawns all ready. She cast a spell to enthrall every new consciousness to enter a world, taking care to choose one of the smaller realms. Not because it was easier; since only so many minds would fall under it at any given time, and each mind would be as tractable as an infant's, the challenge was not in the magic itself. The challenge had never been in the practice of magic, but by its application. So she took on a lesser realm to avoid suspicion of her designs. A realm that had recently experienced its first worldwide war.
According to studies of Midgardian water cycles, the spell (“curse” is such an ugly word - as many spells have gone disastrously awry as curses have turned the tide in crucial battles) should, if dispersed in more than two oceans, have humanity exposed and open to her will just in time for the inevitable coup.
Said coup went poorly (what did he expect, seizing the throne without enlisting her help? and although she knew he only targeted Midgard once Thor formed allies there, she couldn’t help wondering if he - or worse, Odin - had an inkling), with no time to execute her designs. It was not fruitless, however - Amora planted a corollary that would, within a year of the king being deposed, automatically make the humans obey her will.
Amora had not counted on the king being deposed without her realizing it, so a year after Loki took Odin’s place (and face! zing), all humans found themselves subject to a will that didn’t exist. How could she command an army she didn’t realize was ready to follow her? Worldwide, everyone under the age of ninety grew listless in the extreme, performing only enough functions to remain alive and remotely healthy (i.e., ready for battle).
Well. Not all humans.