Hazardous (X)
When a creature with the hazardous X keyword is attacked, it deals X damage to the attacking creature before the fight resolves. (The active player chooses whether this occurs before or after other “Before Fight” effects and keywords.) If this damage destroys the other creature, the rest of the fight does not occur.
If a creature with the hazardous (X) keyword gains another instance of the hazardous (X) keyword, the two X values are added together.
Unofficial FAQ
If I attack a creature that has both Hazardous and Elusive, and the Hazardous destroys my creature that was attacking, has Elusive been turned off, or is it still in effect for a future attack?
Elusive has been turned off. Because you engaged in a fight the Elusive keyword has been triggered and cannot be triggered a second time. [1]
Commentary
If a creature with the Assault keyword attacks a creature who has the Hazardous keyword, the active player chooses which resolves first. If the creature dies to the Hazardous effect before the active player resolves the Assault keyword, the Assault is not applied. Similarly, if the creature dies to Assault before the active player resolves Hazardous, the Hazardous is not applied. This is because Ability disallows either ability from triggering on a card that is not in play.