The background describes how the character lived before the campaign began, and the ones outlined by the Player Handbook come with particular stat changes.
It would only happen through dramatic changes or life events that a character experiences - something so life-changing that their fundamental being is altered. The background and alignment details are less likely to change. Players can reach new levels through gaining experience from each encounter (combat or not), recorded in that sub-heading, or as milestones, which means the dungeon master will instruct the players to level up once they reach certain story beats. The maximum is level 20, which the player can divide into multiple different classes (in order to combine abilities in unique ways) as they choose. The class and level section can and will change as the character progresses through a story (or stories). RELATED: 15 Video Games To Play If You Love D&D
