If Ruined Rex attacks into a Shen and dies because of Hidden Blade, does the Deathknell trigger kill Shen or does he still have 5 might when Deathknell resolves?

To resolve this interaction, we must look at the combat sequence, the timing of Deathknell, and the mechanics of Shield.

The Interaction Sequence

1. Showdown: During the Showdown, Hidden Blade is played to kill Ruined Rex. Ruined Rex is sent to the trash (Rule 428.1.a.1).
2. Deathknell Trigger: Because Ruined Rex has Deathknell, his ability is added to the chain as a pending item before he is moved to the trash (Rule 808.1.d.2).
3. Deathknell Resolution: The chain resolves, and Ruined Rex’s Deathknell deals 4 damage to Shen.
4. Combat Cleanup: Following the Showdown, the combat proceeds to the damage step and subsequent cleanup where all surviving units are healed (Rule 461.1.a.1).

Does Shen have 5 Might?

Yes, Shen will have 5 Might.

According to the Shield keyword mechanics (Rule 814.1.c), Shen gains +2 Might only while he holds the Defender designation.

• Shen enters combat as the defender, which grants him the Defender designation.
• This designation persists throughout the combat phase until the Combat Cleanup step is complete (Rule 461.7.a).
• Because the Deathknell ability of Ruined Rex resolves during the Showdown (before combat cleanup), Shen still holds the Defender designation at the moment the damage is dealt. Therefore, his Shield 2 is active, and his Might is 5 (3 base + 2 from Shield).

Conclusion

When Ruined Rex’s Deathknell resolves, it deals 4 damage to Shen. Since Shen has 5 Might, the 4 damage from Ruined Rex is not enough to kill him.

Note: I could not fully verify this ruling against the retrieved rules.