Kaer Dhu is a ruined, black stone fortress tucked away in the jagged northern mountains of Ard Skellig. This ancient stronghold plays a vital role during the late game main quests involving the elven sage Avallac’h and his secret research on the Elder Blood. The structure features ice-covered elven arches, ancient laboratory foundations, and heavy elemental magical wards, functioning as a high-danger archive where the true cosmic stakes of Ciri’s lineage are uncovered.