Tainted Grail: Kings of Ruin · Board game rules
When can I perform an Opportunity Attack?
Quick answer
The Opportunity Attack isn't something you perform — the Enemy resolves it automatically whenever the Active Character plays no cards at all during their Activation.
Details
Opportunity is a special attack type found on the bottom of the Encounter card. It triggers if, during step 2 (Playing Cards) of a Character's Activation, they fail to play even a single Combat card — whether because they can't or simply choose not to [p.22 — Combat Basic Combat Overview / Encounter Card].
Procedurally, this happens during Phase II of the Combat Turn: if the Active Character can't or doesn't want to play any cards, you resolve the Opportunity Attack shown on the Encounter card, and then proceed directly to Phase III, the Enemy Attack [p.24 — Combat Turn]. This is confirmed in the quick-reference summary as well: "If you didn't play any cards, resolve the Opportunity attack" [p.44 — Character Activation].
Watch out: Playing at least one card during your Activation avoids the Opportunity Attack entirely — it only occurs when zero cards are played, not when you simply play fewer cards than you'd like.