i made a Pyre Elemental of Vengeance the idea came from Ghost rider and kalista from league of legends. The Idea is a true neutral elemental that when someone dies via fire damage from being burned alive at a Pyre that humanoid communes with this elemental and forms a pact, become part of the elemental using your soul as fuel for its fire and merge with it and the elemental will track the other humanoids who killed pact mortal.
now you can come up with any idea for how a Pyre elemental to spawn but the best idea i could come up with is "salem witch trials" the idea that group of witches were gather and burned alive together are they collective need for vengeance fuses their souls and the elemental energy of fire make a Pyre elemental.
yes i know that in older version of DND had a pyre elemental and it was from raven loft and it was evil i wanted something more morally ambiguous nether good or evil just a being of pure revenge.
lastly please let me know what you guys think! i took a long time to think of this monster and i would love feed back! the Stat block i give here is like a Pyre Elemental of Vengeance at max power but CR isnt one of my strongest point in 5E\5.5E i would love to know what everyone here thinks :)