Can my opponent use Sacrifice on Glasc Mixologist and can I react with Star Cross to cancel Sacrifice?
No, you cannot use Star-Crossed to cancel your opponent's Sacrifice.
The Ruling
In Riftbound, "canceling" a spell or ability is not a standard mechanic. The only way to stop a spell or ability from resolving is to Counter it, and Star-Crossed does not have an effect that counters spells.
- Countering: Per Rule 425.2.a, you can only Counter a card or ability when specifically directed to do so by a card effect (e.g., a card that says "Counter target spell").
- Star-Crossed: Its text states: "Return a friendly unit and an enemy unit to their owners' hands." It does not contain a "Counter" instruction. Even if you target the Glasc Mixologist with Star-Crossed, the Sacrifice spell has already been finalized on the chain, and its costs (including killing the Glasc Mixologist) have already been paid (Rule 357.4). Returning the Glasc Mixologist to hand after it has already been killed as a cost does not remove Sacrifice from the chain.
Sequence of Events
If your opponent plays Sacrifice and you respond with Star-Crossed:
- Opponent plays Sacrifice: They pay the additional cost by killing their Glasc Mixologist. Sacrifice is finalized and added to the chain.
- You play Star-Crossed: You add Star-Crossed to the top of the chain as a [Reaction].
- Resolution (LIFO - Last-In, First-Out):
- Star-Crossed resolves: It attempts to return the targeted units to hands. If Glasc Mixologist is already in the trash (because it was killed as the cost for Sacrifice), that part of the instruction will fail to find a valid target and be ignored (Rule 359.3.e.5).
- Sacrifice resolves: Even though the Glasc Mixologist is no longer on the board, Sacrifice has already been successfully played and its costs paid. Your opponent will proceed to draw 2 cards and channel 1 rune.
This is my interpretation based on the mechanics of costs and resolution in Riftbound. No official FAQ exists for this exact scenario, but it aligns with the handling of costs and invalid targets defined in Rules 357.4 and 359.3.e.