How to Beat The Castle's Source of Corruption

"Source of corruption" is the game's category for its bosses — the generic achievement reads "Defeat any boss". The castle's one is the story's named climax: whatever sits at the heart of the Regal Ruin is the corruption the whole map has been leaking.

Launch-week numbers frame the fight: 2.2% of players had both Regal Ruin and The Last Shadow within days — the castle gates the story's ending behind everything the game taught. Expect the full toolkit to matter: rings covering the six parent stats (Sixfold Band), unlocked Supers (Fourfold), and a leveled build.

Arrive with the completionist kit: two rings covering the six parent stats, all four Supers unlocked (Fourfold), quality-20 gear where you can get it, and Veteran-grade habits. The achievements tied to the castle assume a build, not a lucky run.

• Treat the castle approach as the pre-fight: full resources, no skipped sub-zones.
• Spend Supers deliberately — Fourfold exists because the endgame expects them.
• Keep the ring pair's stat coverage active; the six parent stats are the fight's real health bar.
• If the fight stalls, farm run 2+ areas (Return, Bolder) and return stronger — the game is built for re-runs.

The Last Shadow ("Finish the required story") completes with the story — and the game continues: Veteran difficulty (Veteran, 1.1%), run 2+ area clears (Return, Bolder, 0.1%) and every-dungeon run 2 (Monster Hunter, 0.1%) are the post-story mountain.