The lore isn’t fully fleshed out yet (a lot of it pinned on Rytlock’s trip to the mist), but essentially Revenants make a deal with Legendary beings in exchange for tapping their power. Its also heavily implied that a Rev can hear most of them talking, sometimes arguing with each other…. and it takes a fair amount of disciple to sort through them.
Rytlock is the first Rev to use this particular type of magic, despite there a being functionally similar precedent with the Ritutalist. Rits communed with the spirit world in general, which includes mist energy…. but it was mostly with lesser beings, or those with Ancestral ties to Cantha who willingly offer their help. As far as the story is concerned, Rytlock currently represents something like a Prime Revenant….. the “liaison” between the Mist heroes and Tyria. Technically its because hes the only Revenant to officially exist at the moment. Its only been through his efforts that the Legendary figures offer their cooperation. And judging by some dialog, a few of them aren’t entirely happy with the arrangement.
As to why there are so few?…… Revenant’s class structure uses each legend to design a Specialization theme around. This limits them to 1 trait line, one set of utility skills, and favors a specific weapon. As of HOT, when Revs were introduced, each class only had 6 Specializations….. 5 core, and 1 Elite spec. With PoF, each class gets an additional Espec… and will be the pattern for each expansion.
With PoF, Revs are getting “Renegade”, enlisting the help of Kalla Scorchrazor (descended from Pyre Fierceshot). Being famous in Guildwars isn’t really enough to qualify as a Mist Legend; as apparently the way the Mists resonate with reality has the strongest echos in spikes of magical energy, or collectively large emotional energy surrounding specific events. Strong personalities tend to retain them when they pass into the mists… but those who pass during such events seem to be further empowered by it. If you read the plaque on Kalla’s memorial in Black Citadel, you can get an idea of why she made it as a Mist Legend.
Mallyx was a General of Abbadon, and almost ascended to his throne after the God fell. Already residing in the mists when he was defeated by Heroes of Guldwars 1, its presumed he was imprisoned by the Forgotten in the realm of Torment. Its unclear how he would get access to Mallyx if his prison was guarded….. so it could be an indication of a weakened state of the God’s military forces, or he was given access by his Keepers. Either way, its clear that Mallyx was enlisted by force.
Glint on the other hand is obviously willing to help, as was Jalis and Ventari.
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