Ruined Rex

Q. You move Ruined Rex into battlefield against Elder Dragon. Does your Ruined Rex kill the Elder Dragon using its might plus its deathknell ability?

No. Units are healed before deathknell and damage triggers resolve.

Reminder of the combat resolution:

1. Combat cleanup (includes healing all units).
2. Deathknell and damage triggers (such as Imperial Decree) resolve.
3. Combat result is determined.
4. Control of the battlefield is established.
5. Combat ends.