Willowisps are incorpreal swamp spirits created from the vapors that rise from the dense jungle swamps of Kehjistan. Mindless, Willowisps seek out the energy contained in all living things and enjoy ambushing good creatures. While not evil in nature, their feeding habits make them a serious threat to adventurers.
Though they must manifest to attack, Willowisps project a vague, ghostly presence even when incorporeal. When they do manifest, Willowisp attack with either their claws or lightning. As long as they are manifest, they are vulnerable to corporeal forms of attack.
Willowisps are encountered in Act III and Act IV of Diablo II, specifically in the Kurast jungles in the former act, and also in Act V, as guest monsters. When they move around they are virtually invisible, save for a tiny light radius. Before they attack, they become visible again. Willowisps attack in large packs and use Lightning spells. In large packs they can easily confound players and zap them down before even a single Willowisp is killed.
It is best to have some Lightning Resistance when fighting these monsters, along with a large amount of life. As with many enemies, though their attack is powerful they do not have a lot of life. Willowisps also drain mana when attacking.
Due to a bug, their ranged attacks deal additional damage (their melee attack damage added to the spell), making them iconic tough enemies on high difficulties.