In Elden Ring, which boss is fought at Castle Sol?

In Elden Ring, Commander Niall is the boss fought at Castle Sol. The castle stands in the northern part of the Mountaintops of the Giants and is filled with spectral soldiers, spirit birds, and powerful knights.

Commander Niall fights with a large halberd and a prosthetic leg capable of producing electrical attacks. He begins the battle with spectral soldiers, including a swordsman and a shield-bearing warrior. Defeating him grants access to the area containing part of the Haligtree Secret Medallion.

Commander Niall is commonly confused with Commander O’Neil, another commander encountered in the Aeonian Swamp of Caelid. Their names and military titles are similar, but O’Neil’s battlefield is in Caelid, while Niall’s fortress is Castle Sol.