There is a combat between enemy player unit attacking and Ferocious Forerunner, both units have the same might. Defender has Tideturner hidden on battlefield. Both players pass so unit and Forerunner trade. Can defending player play Tideturner after combat, for example on Forerunner's deathknell ability?

Yes, you can play Tideturner as a reaction to the Deathknell ability of your Ferrous Forerunner.

Explanation

According to the Showdown Flow and Chain Rules, triggered abilities like Deathknell use the chain. Because the chain is open for reactions when a triggered ability is placed on it, you are permitted to play Tideturner from its hidden state at that time.

Sequence of Events

1. Combat Cleanup: After combat damage is dealt and units are destroyed, the Deathknell ability of your Ferrous Forerunner triggers and is placed on the chain.
2. Reaction Window: Because the Deathknell ability is on the chain, the state is CLOSED, which allows you to play [REACTION] cards.
3. Playing Tideturner: You play Tideturner from its hidden state as a reaction to the Deathknell ability.
4. Chain Resolution:
   • Tideturner enters play (this happens immediately as it finalizes).
   • Tideturner's "When you play me" ability triggers and is placed on the chain on top of the Deathknell ability.
   • The chain resolves in LIFO (Last-In, First-Out) order:
     1. Tideturner's "When you play me" ability resolves (you perform the swap).
     2. Ferrous Forerunner's Deathknell ability resolves (you create the two Mech tokens).

This is consistent with FAQ #8909, which confirms that Deathknell effects are triggered abilities that resolve normally, and Rule 737.6, which allows hidden cards to be played as reactions whenever a card with Reaction timing could be played.