One of the Eldritch Invocations warlocks can take is Maddening Hex (Xanathar's Guide to Everything, p. 57):
(Prerequisite: 5th level, hex spell or a warlock feature that curses)
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. When you do so, you deal psychic damage to the cursed target and each creature of your choice that you can see within 5 feet of it. The psychic damage equals your Charisma modifier (minimum of 1 damage). To use this invocation, you must be able to see the cursed target, and it must be within 30 feet of you.
The description says "around the target cursed by [hex or a warlock feature]". It does not say "a target" nor does it address the possibility of multiple targets active at a time for you to choose.
If I cast hex on a creature, and curse another creature with Hexblade's Curse, then if I activate Maddening Hex, do both creatures get damaged (and others in their vicinity)?
If this is true, this might be so broken because if the enemies are clumped together they will take a lot of damage!