If Unsung Hero is in my discard pile after dying and putting its Deathknell trigger on the chain and then Spectral Matron goes on the chain, Spectral Matron resolves targeting Unsung Hero. Unsung Hero brings it back to the base, do I still get the Deathknell trigger from Unsung Hero?

Yes, you still get the Unsung Hero Deathknell trigger.

According to Rule 734.1.d.3, when a unit with Deathknell dies, the game notes its location, attributes, and other relevant details before it is moved to the trash. This information is then used to process the trigger once it is finalized on the chain.

Because the Deathknell trigger has already been added to the chain as a pending item and the necessary information (such as whether it was "Mighty" at the moment of death) has been captured, the subsequent movement of the card from the trash to the base by Spectral Matron does not invalidate or remove the trigger from the chain.

Sequence of Events:

1. Unsung Hero dies: The Deathknell trigger is added to the chain as a pending item. The game notes its attributes (specifically its Might at the moment of death) to be used when the trigger resolves.
2. Spectral Matron is played: Its "When you play me" ability is added to the chain.
3. Spectral Matron resolves: You choose Unsung Hero in your trash and play it to your base.
4. Deathknell trigger resolves: The game processes the Unsung Hero Deathknell trigger using the information noted in Step 1. Since the trigger was already pending and the "Mighty" condition is checked based on the captured state at the time of death, the effect will resolve normally.

Note: As per FAQ 4157, the "Mighty" condition is checked when the Deathknell resolves, but it uses the attributes captured at the moment of death. If Unsung Hero had 5+ Might at the moment it died, you will draw the 2 cards. If it had less than 5 Might at that moment, you will not draw the cards.