If a 6 might Stormbringer moves to a battlefield where a 6 might unit with Master Yi Legend (alone) is located, does it kill the Yi unit?

Ruling: Yes, the Stormbringer kills the Yi unit. The unit dies to the marked damage before a combat can be staged and initiated, so Master Yi's passive (which requires the defender designation from an initiated combat) never comes into effect.

Sequence:

• Stormbringer deals 6 damage to the Yi unit during the move action
• In cleanup (322.4), the Yi unit dies from the marked damage
• This happens before a combat can be staged and initiated (322.10 and 322.14)
• Since combat never initiates, the Yi unit never gets the defender designation
• Without the defender designation, Master Yi's passive never activates

Nuances:

• If the Stormbringer had 5 might instead, it would deal 5 damage, move to the battlefield, and then in cleanup the Yi unit would survive. A combat would be staged and initiated, giving the Yi unit the defender designation and activating Master Yi's passive (making it 8 might with 5 damage marked)
• This interaction creates a rules hangup (contested battlefield controlled by a player with no units where neither showdown nor combat can be staged) that wasn't fixed in the Spiritforged rules update