Create Spawn. The body of a creature slain by the abyssal geyser is consumed and is formed into a new demon after 1 minute.
Immutable Form. The abyssal geyser is immune to any spell or effect that would alter its form.
Magic Resistance. The abyssal geyser has advantage on saving throws against spells and other magical effects.
Regeneration. The abyssal geyser regains 10 hit points at the start of its turn if it has at least 1 hit point.
Multiattack. The abyssal geyser makes three melee attacks or two ranged attacks.
Unborn Demon. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 10 (2d4 + 5) slashing damage.
Vile Projectile. Ranged Weapon Attack: +9 to hit, range 30/120 ft., one target. Hit: 14 (2d8 + 5) poison damage.
Abyssal Corruption (Recharge 5-6). Each creature within 30 feet of the abyssal geyser that isn't a demon must succeed on a DC 17 Constitution saving throw or become corrupted for 1 minute (save ends). While corrupted, the creature is poisoned and has vulnerability to all damage.
Spawn Greater Demon (1/Day). The abyssal geyser spawns one demon of CR 6 or lower.
The abyssal geyser can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The abyssal geyser regains spent legendary actions at the start of its turn.
Unborn Demon. The abyssal geyser makes one melee attack.
Spawn Lesser Demons. The abyssal geyser spawns two demons of CR 1, four demons of CR 1/2, or eight demons of CR 1/4 or lower.
Weaken (Costs 2 Actions). The abyssal geyser targets one creature within 30 feet of it that isn't a demon. The target has disadvantage on all saving throws until the end of the abyssal geyser's next turn.
Description
Demons are spontaneously birthed from the matter of the Abyss itself. When enough demonic energy is concentrated into one place, regardless of the plane it is present in, an abyssal geyser is created. An abyssal geyser is a highly volatile coalescence of planar matter that spawns new demons at a significantly faster rate than the rest of the Abyss. They are somewhat uncommon in the Infinite Depths, and they are highly sought out by demon lords that seek to swell their ranks and to deny others from gaining an advantage over them.
Despite their name, abyssal geysers all have unique appearances. Some are cracks in the wall of the Abyss that spew out endless filth that immediately forms into new demons. Others are large pools of bile and poison whose spawn emerge from their infinite depths. Some abyssal geysers are even active, creature-like entities, using tendrils and other natural weapons to defend themselves and their progeny.
Abyssal geysers are considered demonic constructs. Some are naturally birthed by the Abyss, while others are artificially constructed and are used as a staging ground for a demonic incursion into another plane of existence. Either way, these terrain feature-esque entities typically lack mental facilities of any kind, but they instinctively take actions to defend themselves against attackers.
Out of combat, an abyssal geyser produces demons at a much slower pace. In one hour, an abyssal geyser can produce 25 demons of CR 1, 50 demons of CR 1/2, or 100 demons of CR 1/4 or lower.