When a unit or spell has an additional cost of killing a friendly unit, when does Deathknell (or any 'Dies' trigger) go on the chain?

Ruling: For permanents (units), the permanent fully resolves off the chain before Deathknell is put on the chain. For spells, the Deathknell trigger is put on the chain on top of the spell.

Sequence:

• For units: Unit enters play and resolves completely, then Deathknell and "when you play a unit" triggers go on the chain simultaneously (controller chooses order)
• For spells: Spell goes on the chain, then Deathknell trigger goes on the chain on top of it

Nuances:

• Even though the unit dying (from the additional cost) technically happened before the card being played, Deathknell and "when you play a unit" triggers are considered simultaneous for units and the controller chooses the order