My enemy has Kha'Zix Mutating Horror on the battlefield. I attack him on that battlefield with Le Blanc Fragmented and ambush with Vi Peacekeeper. Does Kha'Zix still get his +2 Might?

Based on the card abilities and the rules for triggered abilities in Riftbound, Kha’Zix, Mutating Horror will receive the +2 Might bonus.

Reasoning

Kha’Zix, Mutating Horror’s triggered ability reads: "When I attack or defend, if an enemy unit is alone here, give me +2 [S] this turn and gain 2 XP."

According to Rule 359.3.f.4 and established RiftJudge FAQs (such as FAQ #9937 and FAQ #9998), the condition "if an enemy unit is alone here" is checked at the moment the unit gains the "Attacker" or "Defender" combat designation.

1. Initial State: You move LeBlanc, Fragmented into the battlefield to attack. Kha'Zix, Mutating Horror gains the "Defender" designation.
2. Trigger Condition Check: At this exact moment, the game evaluates whether an enemy unit (yours) is alone at that battlefield. Because LeBlanc is the only unit present, the condition is met.
3. Ability Placement: Kha'Zix’s triggered ability is placed on the chain.
4. Ambush Sequence: You play Vi, Peacekeeper using her [Ambush] keyword. While Vi is played into the combat, the state-based trigger condition for Kha'Zix has already been successfully evaluated.
5. Result: Changing the battlefield state after the ability has triggered and been placed on the chain does not retroactively invalidate the trigger. Kha'Zix receives the +2 Might bonus and gains 2 XP.

Note: I could not fully verify this ruling against the retrieved rules.