The Starscourge Radahn boss fight is done as part of a Festival and, along with Mohg, Lord of Blood in Elden Ring, is one of two bosses you need to defeat to access the Erdtree DLC.
The battle sees the player summon NPCs to beat Radahn as part of the Radahn Festival at the end of Castle Redmane. While you can find this boss pretty early in Elden Ring, it'll probably be way before it's a good idea to try and fight him. This fallen Lord is a serious difficulty spike and you'll need to bring an army with you to take him down. To make the battle ahead easier I'll show you how to beat Radahn in Elden Ring, as well as how to summon the best friends possible to take on the rotted Starscourge himself.
