Death Rune is used in a number of spells such as player killing. Playerkilling spells include vengeance, barrage and burst. By typing "::barrage" in game this will give you 6,000 Water runes 4,000 Death runes and 2,000 Blood runes as long as you have 1B in your pouch or inventory.
Currently, it is dropped by the following Bosses.
Can be obtained from the following Boxes and Chest