Millennia ago (give or take…) a curious cabal of pan-dimensional beings appeared on Earth.

People, being quite self-involved and a bit dumb, didn’t notice. These beings immediately sought shelter.

They tried a plethora of possibilities; caves, trees, shells, postboxes, discarded takeaway boxes, they tried essentially everything.

After a while, they found the perfect place; with us. Not like in a “the strength was in you all along” kinda way, but literally INSIDE human beings. They discovered that a part of human anatomy matched their own with surprising/narratively convenient accuracy.

One simple operation later (1. Pop the top off 2. Scoop out rubbish human skull, 3. Insert new “skull” 4. Say “initiation complete” in a robot-like voice. The fourth step was optional). Then they were set.

Their new homes offered many advantages, not just that they were perfect lookouts for observing this strange little world, but were also pretty darn cosy.

You may assume that people would have trouble without their brains (which were disposed of in the “replacement”), but it turned out that huge chunks of the human race weren’t really using them anyway, so it went unnoticed.

After a few thousand years the beings grew bored of just watching and their desire for power, and mischief, grew. They discovered they could influence the actions of their hosts from within.