If I use Heedless Resurrection and kill Kog'Maw as the additional cost, does the resurrected unit take damage from Kog'Maw's Deathknell?

Ruling: No. The resurrected unit will not take damage from Kog'Maw's Deathknell because the Deathknell resolves before the unit is resurrected.

Sequence:

• Heedless Resurrection is placed on the chain as a pending item
• During finalization, you kill Kog'Maw to pay the additional cost
• Kog'Maw's Deathknell trigger is added to the chain on top of Heedless Resurrection before Kog'Maw moves to trash
• Both items finalize (Heedless Resurrection first per append order, then Deathknell)
• The chain resolves in LIFO order: Deathknell resolves first, dealing 4 damage to all units at that battlefield
• Heedless Resurrection then resolves, placing the chosen unit on the chain as a pending item
• The resurrected unit enters play after Deathknell has already resolved

Nuances:

• You cannot choose Kog'Maw as the resurrection target during finalization because he is still on the board at that point
• Items finalize in the order they were appended to the chain, even though Deathknell is on top