The Mending

It's some time after the defeat of Phyrexia, and Dominaria has a problem: with all the catastrophes throughout the plane's history, an imbalance had built between the multiverse at large and the energies of the planeswalker spark, and it's tearing both Dominaria and the multiverse at large. The plane's resident surviving planeswalkers, making up the bulk of prominent Magic characters at the time, frantically scrambled to reverse the damage, eventually ending in the planeswalker Freyalise sacrificing herself to forge a new balance of the fabric of the multiverse.

The year is 2006, and Wizards of the Coast have a problem: it's been a few years since The Weatherlight Saga ended with a climactic battle for the fate of the multiverse, bringing to end literally years' worth of Magic the Gathering fluff. A few standalone stories followed, bringing some of the game's most popular settings, but without any real overarching plot. You see, the metaplot of Magic fluff relied heavily on planeswalker characters being present, especially as fluff writers wanted to explore planes outside of Dominaria, and since planeswalkers were basically walking gods, this made it something of a challenge to put together a compelling narrative. Their surviving writers, the bulk of prominent Magic writers at the time, frantically scrambled to find a workable plot device, eventually ending in the creation of the Time Spiral block to facilitate a large-scale retcon of the fabric of the multiverse.

Whatever the reason, the result was The Mending: a large-scale rewrite (both in-universe and out) of how the Magic multiverse worked, and the nature of the planeswalker spark in particular. Prior to the Mending, planeswalkers were nigh-immortal, nigh-invulnerable, had incredible levels of magic power, and could shapeshift at will. About the only things that could pose a threat to a planeswalker was another planeswalker or Yawgmoth. Afterwards, they lost the vast majority of their power, including their immortality and invulnerability, with the spark representing little more than their ability to walk the planes in the first place (for context pre-mending planeswalkers where Mary sues to put it bluntly, tbh thats an understatement. planeswalkers pre mending where literally shapeshifting beings of GODLIKE power planeswalers where fucking unstoppable, and the only thing that could stop a planeswalker was, well, another planeswalker.) This allowed Wizards to, for the first time, introduce planeswalkers as a playable card type, and made Nicol Bolas very, very VERY salty (so much so that all of bolas's plans revolve around him regaining his lost strength.)