Chasme

Chasme are large fiends that resemble gigantic mosquitos.

Appearance
Chasmes are bloated, insectoid demons with long proboscises.They appear similar to massive mosquitos, but with human-like hair on their heads.

Abilities
Chasmes make terrible buzzing drones to which demons are immune, but render all other creatures who hear it unconscious unless they pass a DC 12 Constitution save. They are also resistant to spells and magical effects, and can climb walls or ceilings with ease.
 * Melee weapon attack: if a chasme successfully strikes a creature, they take both piercing and necrotic damage, and the creature's hit point maximum is reduced by the amount of necrotic damage until they finish a long rest or are magically restored. If a creature's maximum HP is reduced to zero by a chasme's attack, the creature dies.

History
The Mighty Nein fought chasmes in the Braan. These demons arrived through a portal created by an abyssal anchor, and had previously killed many of the stone giants of Soorna's tribe.