If I use Sacrifice on my Ruined Rex, then Tactical Retreat, how would it resolve, would I get the effects of everything?

Question

Accepted Answer

To resolve the interaction between **Sacrifice** and **Tactical Retreat** targeting your **Ruined Rex**, you will successfully resolve the effects of the **Sacrifice** spell, but you will **not** trigger the Deathknell ability of **Ruined Rex**.

### Sequence of Events
1.  **Preparation:** To successfully replace the cost of the spell, the **Ruined Rex** must already be under the active effect of a previously played **Tactical Retreat**.
2.  **Playing the Cards:** You announce **Sacrifice** and choose your **Ruined Rex** to pay its additional cost ("kill a friendly Mighty unit").
3.  **Paying Costs:** You attempt to pay the cost for **Sacrifice** by killing the **Ruined Rex**.
4.  **Replacement:** The replacement effect from **Tactical Retreat** intercedes. Instead of being killed and sent to the trash, the **Ruined Rex** is healed, exhausted, and recalled to your base.
5.  **Cost Completion:** Per **Rule 357.2.a**, because the replacement effect of **Tactical Retreat** successfully substituted the "kill" action with a "recall" action, the cost to play **Sacrifice** is still considered paid.
6.  **Resolution of Sacrifice:** **Sacrifice** resolves normally. You draw 2 cards and channel 1 rune.
7.  **Deathknell Check:** **Ruined Rex** does **not** trigger its Deathknell ability. According to **Rule 808.1.d.1**, Deathknell only triggers if the permanent is actually killed and sent to the trash. Because the "kill" event was replaced by a "recall" event, the trigger condition for Deathknell was never met.

### Summary
*   **Sacrifice:** Resolves fully (you draw 2 cards and channel 1 rune).
*   **Ruined Rex:** Survives (is recalled to your base, healed, and exhausted).
*   **Deathknell:** Does **not** trigger.

This ruling is based on the interaction between **Rule 357.2.a** (costs replaced by replacement effects are still paid) and **Rule 808.1.d.1** (Deathknell requires the unit to be killed and sent to the trash to trigger). Consistent with **RiftJudge FAQ #9425**, the spell resolves, but the Deathknell trigger fails to activate because the unit was never sent to the trash.

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