It's not surprising that Osiris might rise from the dead.
This Underworld King's rebirth is well-documented in myth and there are quite a few deities in Ancient Egyptian mythology who have been resurrected. It's not an unusual occurrence.
Hearing tales of gods dying and coming back to life might be amusing, perhaps even laughable. You might snicker about the level of such gods. But when you're facing a deity that could potentially revive, it's hard to find it funny.
Although Chen Yu is someone who has met many deities, even personally taken down more than one, the Underworld King Osiris is among the higher beings he's encountered, on par only with Inari God.
The Nine Pillar Gods of Ancient Egypt are all higher beings, with the more powerful ones like God Ra standing even higher.
As for the Underworld King Osiris, although he was killed and dismembered into fourteen pieces by Seti, relying on his wife for revival and his son for vengeance, this does not overshadow his strength.
