If Lee Sin dies from combat damage and stops buffing another unit, causing that unit to have lethal damage marked on it, does the unit die before healing or does it heal and survive?

Ruling: The unit dies before healing. When Lee Sin dies and his might buff is removed, the other unit will have lethal damage and will die during a cleanup check before the healing step occurs.

Sequence:

• Combat damage is dealt to all units
• Units with lethal damage are killed (Lee Sin dies here)
• Lee Sin's might buff is removed from other units
• A cleanup check occurs (or will occur in updated rules) that kills units now at lethal damage
• Healing step occurs

Nuances:

• Under current rules as written, whether the unit dies depends on if there's a triggered ability (like Deathknell) between the killing step and healing step, but this is not the intended design
• The new comprehensive rules will implement cascading cleanups that repeat whenever the game state changes during a cleanup, which will make the unit die consistently
• This ruling parallels how Kog'Maw's Deathknell can kill units between damage and healing
• 0 Might units are intentionally allowed to exist (they only die when they have nonzero damage marked)