Training
Many players have found themselves in the middle of a dungeon when they suddenly have enough XP to level up. So they do so, and for some classes like fighter or rogue that’s not so bad. A fighter gets getter at fighting by practicing, so it’s not unbelievable that they would get better in the middle of a dungeon. But what about other classes, like wizard? How do we explain the wizard suddenly having more spells in his spellbook, despite having no access to a library, or worse yet, ink. Or, even worse still, what if a character multiclasses to wizard? Does a spellbook suddenly appear?
There are many problems with leveling up in the middle of a dungeon. So, what do we do? It’s pretty simple actually, a DM just needs to make a house rule requiring characters to train for a week or so before leveling up. This might mean training with a character of a higher level, or just spending time to practice, but ultimately it can’t be in a dungeon. Essentially this means that the characters need to be in some sort of civilization to level up and need about a week of rest.
Naturally, there are a few things that need adjustments when using this system. As a DM, you might want to be more lenient with the traditional XP rules, given the limited nature of when the players can level up. For example, if the characters earn enough XP to level up twice, or even more times, in one dungeon, you should allow them to do so, even though it is technically against the rules. Also, if the characters are close, like within a few hundred XP of leveling up, and about to head out on a big quest, you might want to allow them to level up early. This does not mean you give them the XP, it just means they level up early and need to make up the XP later.