Monster abilities
A creature's special attacks work like a player's spells. Each one has its own page under Spells with its numbers: dragon breath, beholder rays and the antimagic cone, troglodyte stench, the tyrant fog, and the rest.
Auras
An aura is not cast at you. It sits on the creature and affects everyone inside it, every round. The creature is not affected by its own aura.
For most auras the saving throw DC is 10 + half the creature's hit dice + one eighth of its hit dice. A creature with 40 hit dice has DC 35. Your own level does not change it.
- Aura of Blinding starts from 8 instead of 10.
- Aura of Unearthly Visage starts from 12.
- A dragon's fear aura starts from 15.
- A player carrying an aura adds half their Wisdom modifier.
What happens when you fail the save depends on the aura: fear, blinding, stun, menace, unearthly visage, unnatural, horrific appearance, doom, acid. Several auras have a maximum duration, whatever the creature's hit dice. Aura of Fear, for example, lasts at most 3 rounds (18 seconds).
Boss mechanics are separate
The things a boss does on a timer - pillars, exploding adds, waves - are not auras and not spells. They are on Bosses.