Mastery Trainer
The Mastery Trainer is one of the vendors located at the Inn in Darkest Dungeon II.
This is where the player can spend any 
Mastery Points they have earned during the run on upgrading or Mastering their Heroes abilities.
Mastering Abilities
Every Hero's Abilities can be upgraded one time at the Mastery Trainer for the cost of 1 Mastery Point. This excludes the Bounty Hunter, who has all of his abilities mastered as soon as he is hired.
The player must use all of their Mastery Points at the Inn prior to embarking, or they will be lost. The player will be given a prompt if they try to leave the Inn without spending their Mastery Points.
Mastered abilities will be indicated with the 
Mastery Icon on them.
Resetting a hero's path at the Inn will automatically refund all Mastery Points spent on that hero.