Rule 813.4

Some passive abilities may grant a card or ability Reaction under certain conditions. The card or ability does not have the Reaction keyword unless and until those circumstances are true.

• 813.4.a Those conditions might only be fulfilled while the card or ability is on the chain. In such a case, it can still be…
• 813.4.b If the chain item does not fulfill the conditions by the time step 5: check legality has been reached, the actions…

Official Core Rules