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I'm the DM. During one of our games the party's Cavalier used Unwavering Mark on an enemy. Then the Ranger used hunter's mark (PHB, p. 251) on the same enemy.
The Cavalier's Unwavering Mark feature (Xanathar's Guide to Everything, p. 30) says:
This effect ends early if you are incapacitated or you die, or if someone else marks the creature.
I ruled it as hunter's mark being a "mark" and therefore ending the effect of Unwavering Mark. However, we still are not sure if that is how it is supposed to work.
Is the "mark" the skill refers to another Unwavering Mark from another Cavalier, or does it refer to anything that "marks" a creature?