To fight Starscourge Radahn, start the festival at Redmane Castle in Caelid, speak to Jerren, and use the route behind him to reach the battlefield. A straightforward trigger is to activate a Site of Grace on Altus Plateau. If the castle still looks like a normal dungeon, check that trigger before assuming the encounter is broken. This guide discusses the boss's second phase.
To fight Starscourge Radahn, start the festival at Redmane Castle in Caelid, speak to Jerren, and use the route behind him to reach the battlefield. A straightforward trigger is to activate a Site of Grace on Altus Plateau. If the castle still looks like a normal dungeon, check that trigger before assuming the encounter is broken. This guide discusses the boss's second phase.
Start the festival
One route to Altus is the Grand Lift of Dectus: collect both halves of its medallion to use it. Another route goes through the Ruin-Strewn Precipice; either route has its own dangers. Once on the plateau, activate a Site of Grace. Progressing the Ranni/Blaidd questline can also start the festival; it is not necessary to complete both routes.
Travel to Redmane Castle in southeastern Caelid. When the festival is active, a waygate near the bridge can provide a shortcut into the castle. Find the gathering in the plaza and speak to Jerren above it. After his announcement, go through the building behind him, take the lift down and use the waygate by the shore to enter the arena.
If Jerren is not announcing the festival, confirm you activated an Altus grace or reached the relevant quest step, then return to the castle. Arriving in Altus and merely looking at a grace is not the same as activating it. Avoid assuming a single missing NPC proves the quest is broken; check the plaza and dialogue first.
Cross the battlefield
Radahn opens the fight at range. Watch him rather than sprinting blindly: dodge the incoming attacks and use the golden summon signs scattered in the arena to bring in festival allies. Advance when there is space, and keep an eye on stamina. Torrent can help you cover the distance once you have room to mount; do not assume riding through every projectile is safe.
Once close, let allies share his attention, move out of his weapon swings and take short attacks when they finish. If an ally falls, look for a sign to summon them again. Heal only after creating room; Radahn can change targets and cover ground quickly.
When he disappears
At roughly half health Radahn can launch into the air and return as a meteor. Start moving as soon as he vanishes, scan for his return, and use the wide arena to get clear. After the impact, summon fallen allies again if needed and resume short, cautious attacks. Don't stand still searching the sky while he descends.
If you repeatedly die to the opening arrows, practice dodging before rushing to every sign. If you reach phase two with no healing, shorten your punish windows and upgrade your weapon or increase health elsewhere before trying again. This covers Starscourge Radahn in the base game, not a separate later Radahn encounter.
Sources: Radahn community wiki, festival trigger guide, battlefield guide. These are community and editorial sources; exact encounter behavior should be checked against your installed version.