Ask player 1 has played the card "defy" to counter a spell, player 2 plays discipline and draws their own defy. Can player 2 react to player 1's defy and effectively counter spell it?

Yes, Player 2 can use the Defy they drew from Discipline to counter Player 1’s Defy.

According to the Chain Rules and Riftbound FAQ #950 (and confirmed by similar FAQs like #4838 and #9457), the chain is not a static block. Instead, it resolves one item at a time (Last-In, First-Out), and players are granted priority windows to play new reactions after each individual item on the chain resolves.

Sequence of Events:

1. Initial State: Player 1 plays their original spell (e.g., "Spell A").
2. Player 1's Response: Player 1 plays Defy to counter Spell A.
   • Chain (bottom to top): [Spell A] → [Player 1's Defy]
3. Player 2's Response: Player 2 plays Discipline in response to Player 1’s Defy.
   • Chain (bottom to top): [Spell A] → [Player 1's Defy] → [Discipline]
4. Resolution of Discipline: The chain resolves LIFO. Discipline resolves first. Player 2 gains +2 might for a unit and draws a card, which happens to be Defy.
5. Priority Window: After Discipline finishes resolving, the game enters a priority window before moving to the next item on the chain (Player 1’s Defy).
6. Player 2's New Response: Because it is an open reaction window, Player 2 may now play the Defy they just drew, targeting Player 1's Defy on the chain.
   • Chain (bottom to top): [Spell A] → [Player 1's Defy] → [Player 2's Defy]
7. Final Resolution: Player 2's Defy resolves first, countering Player 1's Defy. Since Player 1's Defy is countered, it is removed from the chain and does nothing (Rule 425.1.a). The chain now only contains the original "Spell A," which will proceed to resolve normally unless countered by another effect.