After going to Altus Plateau,  come to Caelid. Arrive at the Sites of Grace - Impassable Greatbridge, take the Sending Gate to Redmane Castle.

Sites of Grace - Chamber Outside the Plaza. Find Blaidd the Half-Wolf and talk to him.

Boss - Starscourge Radahn. Get the Radahn's Great Rune and Remembrance of the Starscourge after defeating enemies.

Starscourge Radahn - Elden Ring Raiders

Sites of Grace - Starscourge Radahn. Talk to Blaidd the Half-Wolf.