Since it’s a Special Research Task, it should be available forever once you unlock it, so you don’t need to worry about rushing to complete it. Unlike the past Halloween’s “Spooky Message” Special Research Tasks, this set does not focus on Spiritomb. It instead rewards Galarian Yamask.
For those seeking out Spiritomb, you’ll want to check out some of the themed Field Research Tasks available from the ongoing Halloween event.
Pokémon GO, Yamask, Pokémon, Gengar, Niantic
World news – US – Pokémon Go guide: A Spooky Message Unmasked Special Research and Rewards