The Evil Duke is a boss found in the Lumbridge Catacombs and encountered as the elite of wave 10. He has three abilities, Regenerate, a healing ability, and two damaging abilities, Haunting Aura, and Spectral Swipe. You are required to defeat him to complete the quest The Earl of Duke.
As the Evil Duke is a magic-based enemy, he is countered by Ranged. Using the Slayer skill to debuff him is required to cause damage. After he is debuffed the ability slot can be replaced by another attack without removing the debuff. It is recommended to use the Ranged and Summoning skills to damage him and prevent damage.
Players can potentially steal a Bronze crossbow for the off-hand slot from him.