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Maddening Hex (Xanathar's Guide to Everything, p57) refers to Sign of Ill Omen:
As a bonus action, you cause a psychic disturbance around the target cursed by your hex spell or by a warlock feature of yours, such as Hexblade's Curse or Sign of Ill Omen.
In that book I only found Hound of Ill Omen.
It's a Warlock invocation from the PHB. You can find it on page 111.
