Igannok.
Igannok is one of the many runes of the Void Gods. When used, it is said to beckon their attention to the caster. It is wise to only use such marks when the benefits outweigh the potential pitfalls.[1]
It appears in the Chamber of the Guardian in Dalaran as part of 
[45] Dabbling in the Demonic quest.
Patch changes[]
Patch 7.2.0 (2017-03-28): Added.