This publication may no longer be canonical. Stories written prior to The Brothers' War are generally ignored by later writings, and are of uncertain relevance.
The warlord Mandek Ironfist gets the wizardXarl to turn a summonedSerra Angel into a fallen angel with an artifact called the Horned Halo. The angel, Trine, turns into an unstoppable killing machine and Mandek creates an empire. Three centuries later Mandek is long gone, but Xarl is still around, dominating the continent of Corondor thanks to Trine. He appoints his spellsquire Eskil as his heir. The moment Xarl dies, Trine goes on a rampage. Eskil summons another angel, Rahel, who stops Trine. Eskil is struck blind, but becomes a planeswalker.