If Kog'maw attacks a battlefield with a 5 might unit defending it, will the 5 might unit die to Kog'maw's Deathknell after Kog'maw deals combat damage?

Based on the official rules and the specific interaction documented in FAQ #7306, here is the ruling for your scenario:

No, the 5-might unit will not die.

When Kog'Maw attacks a 5-might unit and both die in combat, the game proceeds to the healing step before Kog'Maw's Deathknell ability triggers. This means the 1 damage dealt during combat is healed before Deathknell can resolve, preventing the damage from combining.

Sequence of Events

1. Combat Damage: Combat damage is marked (Kog'Maw takes 1 damage and deals 1 damage to the 5-might unit).
2. Healing Step: Before triggered abilities are processed, the game proceeds to the healing step. The 1 damage marked on the 5-might unit is healed.
3. Lethal Check/Trigger: Units with lethal damage are removed. Kog'Maw dies, which triggers his Deathknell ability.
4. Chain Resolution: The Deathknell trigger is added to the chain and resolves.
5. Damage Application: Deathknell deals 4 damage to the 5-might unit. Because the unit was healed during the healing step, it only has 4 total damage marked, which is not enough to kill a 5-might unit.

Important Nuance

While some earlier rules versions or community discussions suggested that Deathknell resolved before healing, FAQ #7306 and FAQ #1082 explicitly clarify that healing now occurs before Deathknell triggers. This change ensures that combat damage is cleared before triggered abilities resolve.

Note: I could not fully verify this ruling against the retrieved rules.