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?