Planeswalker's spark
From MTG Salvation Wiki
A planeswalker's spark is a latent power inborn into a very small number of sentient creatures across the multiverse. This dormant power can lie within anyone, and only another planeswalker is usually able to detect one. An individual with a planeswalker's spark can live their entire lives without it manifesting, and die as any other mortal; however, under conditions of exteme stress — very often a near-death experience -- the spark can manifest to protect the individual, transforming the person into a powerful planeswalker. This process can be called "Triggering," "Flaring," "Igniting," or "Ascension."
The exact mechanics of the planeswalker's spark are unknown, and many of the descriptions are prerevisionist. A spark cannot be created or passed on genetically; however there is one instance known of a spark being passed on — Glissa's, to Slobad; also, someone can have or be granted planeswalker-like powers, as did Dakkon Blackblade (though there is possiblity that Dakkon always held a dormant spark). Sparks can be abandoned if the individual prefers a mortal life, as was done by Jared Carthalion and Slobad, Greensleeves and potentially Garth One-Eye. The spark itself is intangible, like the soul, though it can be disrupted, as Yawgmoth did to Dyfed. Sparks can be further meddled with, such as being prematurely manifested, as Freyalise did to Jaya Ballard, or kept contained, as Ravidel did to Altair of Coloni.
One theory gleaned from all this is that the spark resides in the planeswalker's "Center of Conciousness," which is usually the brain but can be deliberately placed elsewhere by the planeswalker.
