Can you react with a hidden unit when Wraith of Echoes triggers from a non-Deathknell unit dying?

Ruling: Yes, you can react with hidden units when Wraith of Echoes triggers, even if the unit that died to trigger it did not have Deathknell.

Nuances:

• This follows the same principle as using Deathknell with hidden cards like Kogmaw to retain battlefield control
• The ability to react with hidden cards applies to any reaction window, not just Deathknell triggers