In a recent session, I was confused about how multiple charm effects would interact with each other:
A vampire used its 'Charm' ability to charm a PC. It reads (paraphrased):
The target must succeed on a saving throw or be charmed by the vampire. It takes the vampire's requests in the most favourable way it can. It can repeat the saving throw if the vampire or its companions harm it. Otherwise, the effect last 24 hours or until the vampire is destroyed, is on another plane of existence, or takes a bonus action to end the effect.
Another PC then decided to attempt to wrest control back from the vampire by casting crown of madness on their friend. it reads (paraphrased):
The creature must succeed on a saving throw or be charmed. The charmed target must use its action before moving on each of its turn to make a melee attack against a creature other than itself that you mentally choose. The target can act normally on its turn if you choose no creature, or none are within its reach.
At the time, I ruled that both effects would take place simultaneously. This meant that Crown of Madness took effect first, and the vampire's charm afterwards.
Was this the correct ruling? Can a creature be charmed twice? Or does one charm effect override the other?
Best Answer
Charms don’t override/preempt each other unless that is called out in the rules
The quick answer is no — abilities or spells that impose the charmed condition don’t cancel previous charmed conditions. They can be in effect at the same time.
The answers to this question covered some of these charming rules
One exception is in the case of harpies, once a target is charmed by a given harpy, other harpies cannot charm the creature.
Charms are not all equal
Things get complicated because there is not a single “charm effect.” Various spells or abilities which impose the charmed condition (many of which are called “charm” or include the word in their name) actually impose very different sets of effects (See below).
For example, succubus can command a humanoid affected by her charm ability, while charm person only makes the target regard the caster as a “friendly acquaintance.” In general, the succubus’ commands would take priority over the request made with the benefit of Charm Person.
Charms in direct opposition
The exception to the above is if the command directly contradicts an effect of the other power, or contradicts the charmed condition, for example, if the succubus orders its thrall to attack a wizard who charmed the same humanoid. The charmed condition precludes this (see Appendix A: Conditions, PH, p.290).
In that case, the DM may rule that a contest is in order (See Contests, DMG, p 238) or may rule that the charmed condition precludes the action.
Crown of Madness vs. Vampire Charm
In the question’s particular case, the caster of Crown of Madness could maintain control the charmed character as long as the caster meets the requirements of the spell. That is, choosing a creature for the charmed PC to attack before moving on the first round (which might necessitate the PC attacking an ally) and “using his action to control the target” on subsequent rounds (PH p.230).
Also note the vampire’s effect might outlast Crown of Madness. The spell would persist while the caster’s concentration lasts, up to 1 minute. After that, the vampire’s charm, which lasts 24 hours, would still be in effect.
If the caster ordered the charmed PC to attack the vampire, then that would be a Contest as described above.
Differing Charm Effects
Here are a few charm effects for reference. There are a lot of subtle differences, so you have to be aware of the specifics to determine how they would interact.
Crown of Madness (PH p.229):
Vampire (MM p. 297):
Charm Person (PH p. 221)
Dominate Person:
Succubus (MM p. 285):
Harpy (MM p.181):