Why did Sepulchure put up with Chuckles' laughter? Simple — Chuckles was Gravelyn's first undead minion. "Daddy was so proud!"
Also, why is Artix so afraid of a Fear Feeder even though everyone knows he's afraid of pink? Look at the name of the monster; it feeds on fears! It'll feed on Artix's fear and grow so powerful that not even he, the undead slayer that he is, won't be strong enough to kill it.
Also, why is Artix undead in the Battleoff universe? This is because this is him if he gave into his destiny and became the Champion of Darkness instead of the Undead Slayer we know him to be.
The recent Chronospan quests offer an in universe explanation for the guardian, Dragonlord and Star captain bonus items; The hero was formed out of the merged timelines of the heroes from the other games. he has access to bonuses if the player upgraded in those games because the abilities and powers those heroes possessed (from upgrading) was passed on by the timeline merge