The corruption sigil is an item obtained from the Barrows minigame loot chest. At least one Barrows brother needs to be killed to be able to get the sigil, and killing more brothers will increase the drop chance. The sigil is only dropped when a dragon defender or a better defender is equipped or in the player's inventory. It is possible to obtain multiple sigils per chest.
The sigil is used to create defenders by combining it with new or fully-repaired two-handed Barrows weapons.