Words of Shadow: Volume IV.
Words of Shadow: Volume IV is a book found in the Chamber of the Guardian of Dalaran during the quest 
[45] Dabbling in the Demonic.
Content[]
Words of Shadow: Volume IV
<This book is opened to a page showing a diagram of one of the runes on the ground.>
The rune Xiur can be used to augment many shadow rituals. It is a very powerful rune, and when utilized properly can assist in summoning powerful shadow entities.
For reasons unknown, this rune has adverse reactions with arcane magi, and should not be used in rituals combining the two magic types.
Patch changes[]
Patch 7.2.0 (2017-03-28): Added.