At Battlefield A, my opponent controls 2 units (Mystic Poro and Sinister Poro) and I control 1 unit (Lonely Poro) along with a face-down Hidden Lotus Trap. If combat happens and I activate Hidden Lotus Trap targeting the opponent's Mystic Poro, how does the combat resolve and what is the exact outcome?

All three units die — it's a full trade — and you draw 1 from Lonely Poro's Deathknell. No one scores; Battlefield A becomes Uncontrolled.

You're the defender (your Hidden card means you controlled the battlefield), so your opponent is the attacker with 3 total Might (Mystic 2 + Sinister 1) vs. your 2 Might.

• Their assignment: 2 damage to your Lonely Poro = lethal on its 2 Might.
• Your assignment: With Lotus Trap active on Mystic Poro, you assign only 1 damage to Mystic (it doubles to 2, lethal on 2 Might) and 1 damage to Sinister (lethal on 1 Might). Rule 465.2.c.5 factors Lotus Trap's doubling into the assignment itself, so you don't need to "spend" 2 damage to kill Mystic.

All damage is dealt simultaneously — all three units die. Lonely Poro died alone (no other friendly units there), so its Deathknell triggers and you draw 1. Combat result is No Result (no units remain), and per Rule 466.5.b the battlefield becomes Uncontrolled.

One caveat: Sinister Poro's attack trigger lets your opponent pay [1] to move your Lonely Poro to its base before damage — if they do, the combat never reaches a damage step and Mystic Poro survives while they establish control.