Death by Dying

In most D&D games, the default attitude towards death is that it’s not really much of an impediment after 6th or 7th level. And even prior to that threshold, it’s often possible to be resurrected via a non-player character or MacGuffin. Your character only stays dead if you choose for them to stay dead, which is typically related to having another character concept you want to give a try.

Except when the Dungeon Master says otherwise. 

Tacking restrictions or requirements onto resurrection magic is a common house rule. Such as a special ritual or difficult check that could fail. Or flavour-based restrictions, where only certain people can be raised at certain times, and the magic fails if the gods decide someone’s time has come.

As this is a major change to the assumptions of play, and changes the risks players might take during a session, it absolutely requires a conversation at the table. The players need to be warned that dead is dead or that there is a cost associated with resurrection.