Nihilego (5e Creature)
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Medium aberration, neutral
Saving Throws Con +7, Int +5, Wis +6, Cha +6
Beast Boost. When Nihilego reduces a hostile creature to 0 hit points, the attack bonus for its sludge bomb and the DC for its Dazzling Gleam increase by 3, and the damage for those attacks increases by 5d6. This bonus is cumulative, but resets when Nihilego takes a long rest.
Avoidance. If Nihilego is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.
Magic Resistance. Nihilego has advantage on saving throws against spells and other magical effects.
Sludge Bomb. Ranged Spell Attack: +7 to hit, range 60/180 ft., one target. Hit: 32 (8d6 + 4) poison damage.
Dazzling Gleam (Recharge 5-6). Nihilego shoots a blast of light in a 60-foot cone. Each creature of Nihilego's choice in that area must make a DC 15 Constitution saving throw, taking 52 (15d6) radiant damage on a failed save, or half as much damage on a successful one.
The first spawn of the ultra wormhole to be encountered, Nihilego is a parasite that generates extremely powerful neurotoxins it can blast in the form of attacks. Like most ultra beasts, its power increases as it defeats foes, but it distinguishes itself with its heavy magical resistances.