In Elden Ring, Starscourge Radahn is a formidable demigod who roams Caelid's eastern desert. You must kill Radahn to start the DLC, Shadow of the Erdtree, but before you can challenge him you must activate the Radahn Festival.
Promised Consort Radahn 5 Minute Boss Guide – Elden Ring: Shadow of the Erdtree DLC
When the festival is active, Starscourge Radahn's boss fight will be accessible in Redmane Castle in southeastern Caelid. It's a simple process if you follow our instructions, so keep scrolling.
There are two ways to trigger the Radahn Festival in Elden Ring: reach the Altus Plateau or complete the Ranni the Witch questline far enough until you learn the secrets of Nokron.
I recommend using the Altus Plateau method, as you'll reach the region naturally as you progress through the game. However, if you're pursuing Ranni's quest anyway, that might be the quicker method of the two.
