To defeat Will-o-the-Wisps in The Elder Scrolls IV: Oblivion (including Oblivion Remastered), you need to use specific types of damage since they are immune to normal attacks and poison. The effective methods include:

• Using Silver Weapons : Silver weapons can damage Will-o-the-Wisps, so equipping a silver sword or bow is a good option.
• Using Daedric Weapons : These powerful weapons can also harm them.
• Using Enchanted Weapons : Weapons with elemental or magical enchantments (fire, frost, shock) are especially effective.
• Using Magic Spells : Destruction magic and other offensive spells will work well, and mage characters find these enemies easier to handle.
• Hand-to-Hand Combat : If your Hand-to-Hand skill is at least Journeyman level (50+), you can damage them with bare fists.

During combat, Will-o-the-Wisps drain your health and magicka with a visible energy link. When you see this drain attack, back away until the effect breaks, then close in to strike again. Repeat this hit-and-retreat tactic to avoid losing resources

. Additionally, using the Reflect Spell ability or potions can turn their health/magicka drain into healing for you, providing a strategic advantage

. After defeating them, you can loot Glow Dust , a valuable alchemy ingredient used for effects like Reflect Spell and Restore Speed

. In summary, to defeat Will-o-the-Wisps:

• Use silver, daedric, or enchanted weapons, or magic spells.
• Employ hit-and-run tactics to avoid their drain attack.
• Consider using Reflect Spell to counter their drain.
• Level up Hand-to-Hand if you prefer unarmed combat.

This approach ensures you can effectively kill Will-o-the-Wisps despite their immunities and dangerous abilities