Cleric Forsalle was a high-ranking cleric of Lindelt only mentioned in the Resplendent Life miracle description, a miracle that greatly resembles Replenishment from Dark Souls 1. Replenishment was used by Cleric Knights and it seems Cleric Forsalle followed in this tradition.
Resplendent Life (Miracle)
A great miracle used by highly-ranked clerics. Forsalle was a master of miracles who fought battles across the lands. His allies called him a holy knight, but his enemies feared him for his 'demonic powers'.
It is likely Lindelt, or at least the Archdrake Sect of Lindelt was created by survivors of Shulva, the Sanctum City. It is therefore quite possible that Forsalle was a member of this tradition. Who Lindelt fought against is never explicitly stated, but it is said that its well-honed miracles and unwavering faith in the face of death made them worthy rivals to Drangleic's forces in their day.
Theories go here.