If a player's character is absorbed by a gibbering mouther, do they make death saving throws as normal, or are they instantly killed?

Question

I am a DM for the Tomb of Annihilation published adventure. My group will be exploring an abandoned mine near Fort Beluarian, so I wanted to create a level mini-dungeon for a party of five people.

Choosing the monsters randomly, I put a Gibbering Mouther in the dungeon. However, reading its Bite attack, I read that if the bitten creature drops to 0 hit points, it's absorbed into the Gibbering Mouther.

What does this mean? If the character drops to 0 HP from the attack, are they instantly killed because then they're absorbed into the creature?
Or does that character make death saving throws as normal, and if they pass, they're not absorbed? If so, can the character return to battle if healed?

Which one is it?

Answer 1

A creature is absorbed by the Gibbering Mouth only if it's killed by its Bite attack, not when it drops to 0 hit points.

The Gibbering Mouther's bite attack states:

[...] Hit: 17 (5d6) piercing damage. [...] If the target is killed by this damage, it is absorbed into the mouther.

This means that the creature is not absorbed into the Mouther when it drops to 0 hit points. If the creature is dropped to 0 hit points by this attack, then it follows the usual rules (falls prone, is unconscious, and starts making death saving throws). If the Mouther attacks a creature at 0 hit points and makes it fail its 3rd death saving throw, or deals massive damage to the creature, killing it, only then it is absorbed.

Note: this is only usually relevant to player characters. NPCs and monsters typically die outright when dropped to 0 hit points (and thus would be absorbed when dropping to 0 hit points), unless the DM decides to have them fall unconscious and make death saving throws as well.