Drained
This skill die's ability is resolved instantly as described in its entry. Then, that die is placed on the adventurer'sAdventurerEach character created by a player is an Adventurer. Each adventurer is a party member. race sheet. It is unavailable for the remainder of the battle and is considered drained.
Recover Dice section in rulebook (pg. 23) re: drained dice
This section says that you can recover drained dice during the Adventurers Rest step of the End of Day PhaseEnd of Day PhaseActivation of any province effect on the overland map is resolved. Then, the party rests and may trade items..
This should instead say you recover drained dice during End of Battle CleanupEnd of BattleA battle ends after the current round when any of the following occur: The party achieves the encounter's objectives, or the party retreats from battle, or all adventurers are defeated.. No drained dice recovery happens during Adventurers Rest step.
SourceIs it possible to drain a status or fatigue die?
When given the option to drain dice, only skill dice can be selected.
Source