If I move my Blazing Scorcher to a battlefield my opponent controls, and they use Retreat to return their only unit back to their hand, will they still control the battlefield for Ahri's legend ability effect, or will my unit remain 5 might instead of becoming 4?

Ruling: The opponent will control the battlefield until the showdown ends (when both players pass focus in sequence), so your 5 might unit will become 4 might. Additionally, Ahri's legend ability is a triggered ability that gives your unit -1 might the moment you gain the attacker designation and the ability on the chain resolves.

Sequence:

• You gain the attacker designation
• Ahri's triggered ability goes on the chain
• The ability resolves, giving your unit -1 might
• The opponent controls the battlefield until the showdown ends (both players pass focus in sequence)

Nuances:

• The -1 might is applied when the triggered ability resolves on the chain, not instantly when you gain attacker designation
• Battlefield control persists through the showdown even if the defending unit is removed via Retreat