Mohg, Lord of Blood, resides deep within the Mohgwyn Palace. There are two primary ways to reach him: either by using the Waygate located in the Consecrated Snowfield (specifically, the western part) or by following White Mask Varre's questline until he grants you the Pureblood Knight's Medal, which teleports you directly to the palace. Mohg is a challenging boss known for his bloodflame attacks and the 'Nihil' ritual. It is highly recommended to acquire Mohg's Shackle, found in the Subterranean Shunning-Grounds, as it can stun him twice in his first phase. Additionally, the Purifying Crystal Tear, obtained from Eleonora, Violet Bloody Finger, can negate the damage from his 'Nihil' ritual, making the fight significantly more manageable. Focus on high-damage output and maintaining distance during his bloodflame phases.
