Death Runes
A death rune is a rune that can be used as either unholy, frost or blood. It has lower priority than your other runes, meaning that if you have a Frost rune and a Death Rune available and cast Icy Touch, it will choose your Frost Rune first.
For one thing, death runes are special in that they Maple Story Mesostemporarily replace one of your normal runes and allow you to utilize it to power your abilities, as if it was a rune of any needed type. Likewise, they allow a great deal of flexibility in your combat rotations by allowing you to swap weaker attacks for stronger ones or giving you more chances to utilize your reactive abilities.
