If I attack with Kha'zix Mutating Horror, and the opponent reveals a unit in response to the attack trigger (making it two units), do I get the +2?
Yes, Kha'Zix, Mutating Horror will receive the +2 Might bonus and gain 2 XP.
According to Riftbound rules and established rulings (specifically RiftJudge FAQs #9849, #10198, #10465, and #10176), the condition "if an enemy unit is alone here" is evaluated at the exact moment the unit gains the "Attacker" or "Defender" combat designation.
Chain Sequence
- Attack Trigger: You attack with Kha'Zix. He gains the "Attacker" designation. At this exact moment, the game checks the condition: Is an enemy unit alone at this battlefield?
- Condition Met: Since your opponent only had one unit at that battlefield at that specific moment, the condition is satisfied. Kha'Zix’s triggered ability is placed on the chain.
- Reaction: Your opponent plays a unit (via flipping it face-up, as specified in your prompt). This adds their unit to the chain, resolving above your trigger.
- Resolution (LIFO): The opponent's unit enters the battlefield. Then, Kha'Zix's triggered ability resolves. Because the condition was already successfully met at the time the ability triggered and was placed on the chain, the subsequent change to the battlefield state does not retroactively negate the trigger.
As stated in the referenced FAQs, the game does not re-evaluate the "alone" condition when the ability resolves; it only checks if the requirement was met at the time of the trigger.