"It's what makes a Planeswalker a Planeswalker," Liliana said. "A mote of the Blind Eternities themselves, stitched into our souls. They are exceedingly rare; whole generations across an entire plane can live and die without a single child being born with a spark. And of those few who do have a spark, only a fraction of them 'ignite'. Those that do suddenly find themselves on a whole different plane with more power than they've ever felt before thrumming through their veins. Before, we were even more than that. We were ageless and immortal, able to create new realities and populate whole worlds with plants and animals and people."

Gods, basically. Some literally, not in that they were any more divine, simply that many Planeswalkers had created worlds upon worlds of people to worship them.

"Now, though, most Planeswalkers are simply powerful mortal mages that can walk through the space between realities - that's what the Blind Eternities are - with a couple of handy tricks up our various sleeves to help us blend in with the population better, aging and dying like any other mortal."

And some of them said, 'No thanks,' and sold their souls to demons about it.