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.
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.