In the D&D 5e game I am DMing, one of my players (level 15 fiend patron warlock) successfully cast dominate monster on a hezrou and compelled it to tell the player its true name. Now the player has the demon’s true name.
I had not given any prior indication as to the power of a true name. The player acquired the true name by his own initiative. We’ve been operating off the common wisdom that true names grant some measure of control over a demon. The dominate monster spell hasn’t ended yet, so we haven’t used the true name yet, but it would be good to know what the options are for using true names once that spell ends.
What can a player do with a demon’s true name, particularly if the demon is already on the Material Plane?
I know that true names are useful for summoning spells such as gate and summon greater demon, as discussed here. But those aren’t helpful if the demon is already present. Traditionally, true names grant a large degree of power over fiends and are guarded jealously, but in 5e it appears that true names aren’t that useful outside the summon greater demon spell from Xanathar’s Guide to Everything.
If it helps, this warlock player has an archdevil as his patron. This archdevil can hold the true name and threaten its release or use if the hezrou does not comply with the player’s demands (which is harder to subvert than the player threatening release of the name by his own power). While the threat of a true name being released is also one traditional form of leverage over a demon, true names need to actually be useful and dangerous for this threat to have any value.
In 5e, the only mechanical effect is when you summon them
The only entry that talks about a demon’s true name outside of the spells you mention comes from the Monster Manual entry on demons:
DEMON TRUE NAMES
Though demons all have common names, every demon lord and every demon of type 1 through 6 has a true name that it keeps secret. A demon can be forced to disclose its true name if charmed, and ancient scrolls and tomes are said to exist that list the true names of the most powerful demons.
A mortal who learns a demon’s true name can use powerful summoning magic to call the demon from the Abyss and exercise some measure of control over it. However, most demons brought to the Material Plane in this manner do everything in their power to wreak havoc or sow discord and strife.
The highlighted portion refers pretty clearly to spells like summon greater demon which say:
You utter foul words, summoning one demon from the chaos of the Abyss. You choose the demon’s type, which must be one of challenge rating 5 or lower, such as a shadow demon or a barlgura. The demon appears in an unoccupied space you can see within range, and the demon disappears when it drops to 0 hit points or when the spell ends.
[…]
At the end of each of the demon’s turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you.
As opposed to prior editions, 5e does not appear to outline any mechanical benefits to knowing a demon’s true name except in the process of summoning them to allow a greater chance of controlling them when that happens. Any additional effects are not even hinted at.