On this page of the Tunic guide, you learn what you can do after finishing the game (end game) and whether it is possible to start the new game plus mode, which allows you to replay the game while keeping your equipment.
Whether you had a Bad Ending or a True Ending, you'll see a message with two options after the credits. The first option, "Retry", allows you to return to the game right before the boss fight in order to find the remaining pages of the Instruction Manual or other hidden collectibles. Choosing this option will require you to face (or meet) the final opponent, The Heir, once again.
New game plus
The second option, "New Game", allows you to start the game in new game plus mode. That means starting the adventure from the beginning but keeping most of the items you collected (weapons, disposable items, or crystals). However, you won't keep the story-related items like keys and artifacts from the Hero's Grave. You have to find them again during the game. Remember that the game's difficulty level will slightly increase if you decide to use the new game plus.