The Evil Mask is an item in Otherworld Legends that can only be obtained by merging three items of your choosing.
While this item is equipped, if your health is damaged such that it would drop to 0, you will instead remain at 1 HP and enter a fearless state for 15 seconds. While in the fearless state, you take no damage but are also incapable of healing (you can still pick up Health Potions but they will have no effect), become larger, remain in a permanent unflinching state and deal +50% damage. This allows you to not only escape death but also get a comeback on your enemies. This item, unlike the Green Mushroom or Sacrifice Backpack which have similar functions, does not get consumed after use: whenever you reach full health again, this item will be able to activate again, meaning you'll effectively get at least one usage of the Evil Mask per scene and more if you can heal back up to maximum health during a battle. The Evil Mask straight up does not activate if you have Hearted Shroud or Paper Armor equipped.
This item is extremely powerful due to it preventing you from dying and its ability to trigger several times, even in the same scene, but it has one big downside: the fact that it's hard to find. There are several items one can get by merging three random items, meaning it's very hard to get this specific item intentionally.