If I'm attacking into a battlefield, with Vi stunning their Watcher and the Might of Our Combined Units each is 15, what happens?

Here's the ruling based on the scenario:

The attacker kills all defending units and conquers the battlefield.

Here's why:

1. Vi stuns the Watcher — Vi's "When I attack" trigger resolves and stuns one enemy unit. Per Rule 423.1.b, a stunned unit does not contribute its Might to combat damage. So the defender's effective damage output is 15 minus the Watcher's Might (e.g., 8 if Watcher is 7 Might, or 10 if 5 Might).

2. Combat damage assignment — The attacker has 15 total damage to assign. The defenders have 15 total Might spread across their units (including the stunned Watcher). The attacker assigns lethal damage to each defender — 15 damage vs 15 total Might is exactly enough to kill them all. The defender assigns their reduced damage to your attackers.

3. Combat Cleanup (Rule 461) — All units heal. Then: if any defenders remain, attackers are recalled. But since you dealt lethal to all defenders, none remain.

4. Result — Only attackers survive → you Establish Control and Conquer the battlefield (provided you haven't scored it this turn).

One caveat: This depends on the defending units' individual Might values. If the defender's units have abilities like Tank (forces assignment priority) or if their Might totals don't break evenly into lethal chunks from 15 damage, the exact math matters. But with equal totals (15 each) and no defensive abilities messing with assignment order, the attacker cleans up and wins the battlefield.