Gloomhaven: Jaws of the Lion · Board game rules
When do you apply perks to your character?
Quick answer
You apply a perk as soon as you earn one—right after completing a scenario or leveling up—by choosing an effect and permanently modifying your attack modifier deck.
Details
Perks are granted as a reward for successfully completing a scenario [p.24 — Perks]. When you receive a perk, you choose one effect from the list on your character sheet, check the box next to it, and apply that effect permanently to your attack modifier deck (removing bad cards and/or adding good "Halt!" cards) [p.24 — Perks]. This is a permanent change, so you should choose carefully [p.24 — Perks].
Perks can also be earned in other ways later in the campaign: every three battle goal check marks unlocks a new perk [p.25 — Battle Goals], and leveling up also grants one perk as part of the leveling-up process [p.29 — New City Interaction: Leveling Up]. In all these cases, the perk is applied at the same point it is received—during the between-scenario bookkeeping, before starting the next scenario.
Watch out: Since perk effects modify your attack modifier deck permanently, make sure to apply them before shuffling or using that deck in your next scenario, as the change cannot be undone once selected [p.24 — Perks].