You can choose to add Godzilla to any game. He isn't an enemy; investigators cannot interact with him. Think of him more as an uncontrollable force of nature. He starts in his Unruly Behavior phase, and every turn you roll a die to determine his actions. He might roar, causing investigators to lose sanity, stomp around, killing enemies and leaving Debris tokens that hinder investigators' movement, or even use his atomic breath to place fire and inflict wounds on all figures in a single zone.
Once the Elder One is summoned, Godzilla shifts to his Territorial Behavior. Things escalate as investigators witness a Kaiju battle between Godzilla and the Elder One unfolding before them. Godzilla still acts every turn, but now he moves towards the Elder One to confront him. However, the Elder One doesn't just take attacks passively; every time Godzilla acts, the Elder One strikes back, either physically or mentally, affecting Godzilla's sanity. Godzilla doesn't have a Health track; instead, each time the Elder One hits him, he becomes distressed. When the Distress track reaches its maximum, Godzilla gives up and retreats, causing the investigators to lose the game. Therefore, when Godzilla begins fighting the Elder One, investigators must assist him because if Godzilla retreats before all stages of the Elder One are defeated, they lose.

CTHULHU: DEATH MAY DIE - GODZILLA (ENGLISH) - FEAR OF THE UNKNOWN is categorised BOARDGAMES / ADVANCED GAMES / COOPERATIVE GAMES and sold by l'Imaginaire, a store and specialised website.