Planeswalker's spark

A planeswalker's spark is a latent power inborn into a very small number of sentient creatures across the Multiverse, which if activated permanently turns the bearer into a planeswalker. A planeswalker is granted the ability to travel between the planes of the Multiverse. The exact mechanics of the planeswalker's spark are unknown, and many of the descriptions are prerevisionist.

Prerevisionist spark description
Before the revisions to Magic's canon, there were several disparate depictions of planeswalkers and their powers. The first Magic novel, Arena, depicted the spark as able to be conceived through incredible amounts of mana being hoarded. A different interpretation to reconcile Arena's depiction into revisionist thought is; the planeswalkers had the spark all along and the mana merely triggered it. Depicted in the novels are instances of planeswalkers giving their spark up to live mortal lives, such as Jared Carthalion, Greensleeves, and possibly Garth One-Eye. One can have or be granted planeswalker-like powers, as was the case with Dakkon Blackblade (though there is the possibility that Dakkon always held a dormant spark).

Revisionist spark description
Sparks occur randomly in any sapient population but not through genetics, nor can they be created artificially. A spark can be magically transferred to another, as with Glissa's spark to Slobad or used up to perform magical works such as Slobad healing Glissa. Sparks can be contained magically, as Ravidel did to Altair of Coloni or Yawgmoth did to Glacian. A spark is intangible and a part of a being's soul. A spark's link to the physical world is sometimes described as a planeswalker's "center of consciousness."

More recent material indicate that the spark is what allows planeswalkers to use mana from multiple planes; should a spark be lost, inactive or defective, they are only able to use mana from the plane they are on. This is evidenced by Ob Nixilis' re-ignition, as he narrates that he can use the mana from other planes. This contradicts older cases of non-planeswalkers being able to siphon mana from multiple planes, such as multiple members of the Weatherlight Crew.

Based on older internal documents, Kelly Digges and Ethan Fleischer described the planeswalker's spark as a little bit of aether, or the Blind Eternities, which is imprinted on a soul as it becomes embodied in a being. A birthmark on the soul, left by aether. The theory is that the lifeforce of all beings is "tethered" in the aether and that when a sentient being's life begins, there is a remote chance that the being's lifeforce will be infused with the essence of the Blind Eternities themselves. In other words, the spark is an infinitesimally small fragment of the Blind Eternities that resides within a being's "soul." That metaphysical connection to the Multiverse is what enables planeswalking for the rare few who come to know how to tap into it.

Additional powers
Before the Mending, those with an activated planeswalker's spark had additional powers; they were almost immortal, invulnerable, granted vast magical power, and could shapeshift. While this power could lie within anyone, it was usually undetectable except by another planeswalker. Planeswalkers rarely died, except at the hands of other planeswalkers.

After the Mending, the nature of a planeswalker's spark changed: they lost their invulnerability, immortality, shapeshifting and most of their additional magical power. Although some planeswalkers, such as Liliana Vess, reacquired some of these properties through other means, these properties are no longer provided by the spark itself. In spite of this, the spark still gives its owner increased magical power and skill; Teyo Verada, once a rather poor shieldmage on his homeworld of Gobakhan, found that his spellcasting became much stronger and more intuitive after his ignition.

War of the Spark
During the War of the Spark, Nicol Bolas claimed dominion over Ravnica and performed the Elderspell that harvested sparks from the many planeswalkers on the plane, which would grant him "ultimate power" and godhood. Spark harvesting by his minions, the Eternals, is lethal. In the end, he was desparked himself and sealed away by Jace and Ugin.