Stag Lord is an enemy in Pathfinder: Kingmaker.
The son of the now-mad druid, Nugrah the Stag Lord was once a child cowed by his abusive father. His story is revealed to the player through a series of visions and notes left by Nugrah at the Abandoned Hut. It would seem that the Stag Lord's mother died in childbirth, causing Nugrah to grow bitter and spiteful of him. Years of torture and torment at his father's hands caused the boy to become susceptible to outside influences. Eventually the boy, now a man, came to Nugrah calling himself the Stag Lord, and he overpowered and beat his father, capturing and tormenting him at the Stag Lord's Fort, where the mad druid is ultimately slain by the player after they open his cell.
As far as the player need be concerned, the Stag Lord is just another bandit chief who is killing and extorting the people of the Stolen Lands. This drew the attention of the Aldori Swordlord Jamandi Aldori, causing her to seek out groups of adventurers to tame the Stolen Lands. As part of a deal to grant the adventurers noble titles and lands of their own, with Restov's support, the Stag Lord needed to be removed. Actually reaching the Stag Lord requires the player to jump through some hoops to locate his fort, unintentionally learn more about his tragic backstory, and finish off Tartuccio in A Bitter Rival before they can travel to his fort.
The Stag Lord is spoken of quite frequently throughout first chapter of the game and is the subject of Stolen Land and Riverbed Nightmare, but actually has very little interaction with the player. Beyond the combat encounter with him in the Stag Lord's Fort, there is a tangential interaction in that the player can poison the Stag Lord's Wine at Thorn Ford, giving him the Sickened condition during his combat encounter.