Fiers

Fiers was a dwarf, revered as a god by Dominaria's dwarves and the Barbarian tribes on the Otarian continent of that plane. He was summoned by Karona as the deity of red magic, and might have been a planeswalker.

Myth of Magic
In "The Myths of Magic"'s short story The Lady of The Mountain, he is portrayed as a god who helped Gaea create the world. He created the dwarves of the plane on his own and also helped create the Lady of the Mountain. The tale states that he is worshipped by all dwarves. However, only a few dwarves other than Otarians appear in the short story, so it is hard to say whether or not this is true. Fiers is repeatedly mentioned by Balthor and Jeska in their speeches indicating that they are believers in him, or that expressions using his name are common in their culture.

Scourge continuity issue
Fiers, along with several other popular characters, are part of a heavily-debated continuity issue concerning the Scourge novel. While Karona summoned Fiers, Multani, Teferi, Lowallyn, and Yawgmoth, that meeting was later retconned without further explanation. It is possible that Fiers is indeed a planeswalker, but if the retcon is to be believed, the real Fiers has yet to appear in the storyline.