If my opponent controls one battlefield with Cythria and scores 7 points, then comes to my battlefield and I use Ride the Wind to send Mundo to their battlefield, does my opponent control both battlefields and win?

Ruling: Your opponent wins by scoring both battlefields, not simply by controlling both of them. When you send Mundo to your opponent's battlefield (where Cythria is) using Ride the Wind during combat, this will most likely result in your opponent gaining control of that battlefield and scoring it after both players pass focus, giving them victory by scoring both battlefields.

Sequence:

• Opponent controls battlefield with Cythria and scores 7 points
• Opponent comes to your battlefield
• You use Ride the Wind to send Mundo to opponent's battlefield during your focus in combat
• After both players pass focus, combat resolves
• Opponent gains control and scores your original battlefield
• Opponent wins by scoring both battlefields

Nuances:

• Victory comes from scoring both battlefields, not merely controlling them
• The timing of when control/scoring happens is after both players pass focus in combat