Glyph of the Terrorguard

Glyph of the Terrorguard is a cosmetic warlock glyph that would cause the player's Summon Doomguard to summon a Terrorguard instead. It doesn't seem to have been implemented.

Source
This item likely would have been created using Inscription (1); taught by Technique: Glyph of the Terrorguard, which has no known source.