Errant-Questors are Stormcast Eternals that undertake perilous quests but not on the order of Sigmar (as Knight-Questors) but on their own ways. They may receive a vision in which they see themselves defeating some graet foe, or be the lone survivor of a Stormcast Eternals army, seeking vengeance while their comrades still remained in Azyr waiting the Reforging.
Those who find themselves on Errant-Questor's path fall into fugue and placed in a cage in Sigmaron. Some wake up and return to their responsibilities with their brethren warriors, their desire to take revenge decreased. Other, however, is awakening to be in armor of Errant-Questor, his gilded plates in the image of lions, and halo lightning and sunbeams shine behind his head. Before him lies the rune-encrusted greatblade - his new weapon and the embodiment of Sigmar's wrath. How these gifts are granted is a mystery since no one can enter the cell of an Errant presumably, but many believe these artefacts are a gifts from Sigmar.
It doesn't matter who he was before it, Errant-Questor now selected and must fulfill his new mission.