Does Targon's Peak ability create a window to React?

Yes, Targon's Peak creates two distinct windows where you can play cards with the [Reaction] keyword:

1. When you conquer: Immediately after you conquer Targon's Peak, its "When I conquer" triggered ability is placed on the chain. Players may play [Reaction] cards at this time, before that trigger resolves.
2. At the end of the turn: When the delayed ability created by Targon's Peak is added to the chain during the Ending Phase, a second window opens for players to play [Reaction] cards before the rune-readying effect resolves.

According to RiftJudge FAQ #2905, while these windows allow for [Reaction] cards, neither reaction point can prevent the conquering player from readying their runes once the respective ability resolves.

Summary of Timing

• Triggering: When you conquer, the "When I conquer" trigger goes on the chain.
• Reaction Window 1: Players can react to the trigger above.
• Resolution: The trigger resolves and sets up a delayed trigger for the end of the turn.
• End of Turn: At the start of the Ending Phase, the delayed trigger enters the chain.
• Reaction Window 2: Players can react to the delayed trigger above.
• Final Resolution: The delayed trigger resolves, allowing the player to ready up to 2 runes.

Note: As established in RiftJudge FAQ #9525, these abilities are independent of the card itself; once triggered, the delayed effect will persist and resolve even if Targon's Peak is removed from the battlefield before the end of the turn.