What happens if I attempt to use Sacrifice while only controlling a single mighty unit under the effects of Tactical Retreat?

To determine what happens when you play Sacrifice while controlling a single mighty unit under the effect of Tactical Retreat, we must look at how costs and replacement effects interact.

The Ruling

When you play Sacrifice, you must pay its additional cost: "kill a friendly [Mighty] unit." Because Tactical Retreat creates a replacement effect that intercedes when that unit would die, the unit is saved (healed, exhausted, and recalled to your base) instead of being killed.

The result is that you successfully pay the cost and play the spell, but the unit is not killed.

Sequence of Events

1. Playing the Spell: You announce Sacrifice and choose your mighty unit as the target for the "kill" cost.
2. Paying Costs: You attempt to pay the cost by killing the unit.
3. Replacement: Tactical Retreat's replacement effect intercedes. Instead of the unit going to the trash, it is healed, exhausted, and recalled to your base.
4. Cost Completion: According to Rule 357.2.a, costs that are replaced by replacement effects are still considered paid. Therefore, the cost for Sacrifice is successfully paid.
5. Resolution: Sacrifice resolves. You draw 2 cards and channel 1 rune.

Summary

You successfully play Sacrifice, draw your cards, and channel your rune. Your unit survives the process because Tactical Retreat replaced the "kill" action with a "recall" action.

Note: This interpretation is based on Rule 357.2.a, which explicitly states that costs replaced by replacement effects are still considered paid. This is distinct from the interaction described in FAQ #9401 regarding Baited Hook, because in that case, the "kill" was an instruction within the card's effect, not a cost paid to play the card.