Arcana Cards may be played at any time and may interrupt the normal sequence of play, however some are only appropriate or useful at specific times e.g. during a Combat Phase or in exchange for additional Cultists during the Spawning Phase.
If a card is played and its effect cannot be applied, it is immediately discarded face-up onto the Arcana discard pile. Once the effect of the card has been applied, the turn resumes from where it left off.