The party tonight was about to face a Demilich. They didn't have any specific knowledge of demiliches, but it was clear they were about to confront the boss, and just about every minion they had faced in the lair so far had either caused fear or was more powerful against frightened targets, so the Bard began playing a countercharm:
At 6th level, you gain the ability to use musical notes or words of power to disrupt mind-influencing effects. As an action, you can start a performance that lasts until the end of your next turn. During that time, you and any friendly creatures within 30 feet of you have advantage on saving throws against being frightened or charmed.
All of the party was in range of the countercharm when the Demilich began to Howl:
Howl – Recharge 5. The demilich emits a bloodcurdling howl. Each creature within 30 feet of the demilich that can hear the howl must succeed on a DC15 Constitution saving throw or drop to 0 hit points. On a successful save, the creature is frightened until the end of its next turn.
Does the countercharm give advantage on this save?
Is it a 'save against being frightened' if you are frightened only when you succeed on the save?
Is it a 'save for being frightened?'
Or is it a 'save against dropping to 0 hp' and advantage from countercharm does not apply?
Best Answer
A really strict reading of just the stats block implies that countercharm cannot help in this case.
A Bard's countercharm grants advantage to saving throws against being charmed or frightened. The saving throw against the Demilich's Howl ability is against dropping to 0 HPs, hence it is a different effect than the two that countercharm protects from. Only in the case of a successful save does the frightened condition apply, but then the saving throw has already been done and there's no application of advantage.
The description text of the Demilich helps here: countercharm actually grants advantage.
The description of the Demilich reports that (emphases mine):
This bit of text clearly states that the demilich's ability is a terrifying howl, which kills the weak-hearted (i.e., fearful) and leaves in terror those who survive: it is an ability based on fright. Since the description is part of the monster statistics, a DM can conclude that countercharm actually does provide advantage against the Demilich's Howl.
Credits to Molot for the basis of this answer, provided in the comments to the question.