From Archon Arcana - The KeyForge Wiki
When a creature with the hazardous X keyword is chosen to be fought, it deals X pending damage to the opposing 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.
—KeyForge Master Rulebook v17 April 2023
If a creature with the Assault keyword attacks a creature who has the Hazardous keyword, the active player chooses which keyword resolves first. If the creature is destroyed by the Hazardous effect before the active player resolves the Assault keyword, the Assault is not applied. Similarly, if the creature is destroyed by the Assault keyword before the active player resolves Hazardous, the Hazardous is not applied. This is because the rules for Ability disallow either ability resolving on a card that is not in play.