Vivziepop’s two hit animated series, Helluva Boss and Hazbin Hotel, exist in the same fictional universe, colloquially known as the Hellaverse. This overlap between the two musicals has created the perfect opportunity for Hazbin Hotel Easter eggs to exist in Helluva Boss, and vice versa. Vivziepop has certainly delivered on this score, with character cameos, subtle background imagery, and many other fun references made between the Helluva and Hazbin worlds.

https://screenrant.com/helluva-boss-hazbin-hotel-easter-eggs-list/