The realm of Mephisto is but one of many "death dimensions". Time moves much more slowly here, a fews days on Earth is like many years here.
Forced to fight a duel against his adoptive father Crash Simpson, Ghost Rider and Crash were transported to Hell to battle each other. However, when Ghost Rider gained the upper hand, Satan sent demons to help Crash win the fight. When it came to delivering the killer blow, Crash couldn't do it, and Satan sent a demon to kill them both. Crash sacrificed himself to kill the demon, and Ghost Rider left his body with a robed figure who was apparently against Satan, returning to Earth.
Deadpool would later kill his friend Micheal so he could tell Mephisto that his underling Vetis was stockpiling power in mortals so he could collect it later and overthrow him.