When Cull the Weak is played and the opponent responds with Discipline on their unit, after Discipline resolves and they draw a card, does the opponent get priority again to play a card they just drew (like Defy) before Cull the Weak resolves?

Ruling: Yes, after Discipline resolves, the opponent (as the controller of the next item on the chain) gains priority again and can play reaction spells like Defy before Cull the Weak resolves.

Sequence:

• Cull the Weak is played
• Opponent responds with Discipline
• Both players pass priority
• Discipline resolves and opponent draws a card
• Opponent gains priority again (as controller of the next item on chain)
• Opponent can now play the Defy they just drew
• Chain continues resolving after both players pass again

Nuances:

• After each item resolves on the chain, both players can react again
• The controller of the newest item on the chain gets priority first
• Players can also hold priority to cast multiple spells in succession