If my opponent has Vex Apathetic at a battlefield and I play Bard Mercurial, can I move him into an open battlefield?
Yes, you can move Bard, Mercurial, but whether you can move him to an open battlefield depends on where you play him.
According to the abilities of both cards, here is the breakdown of what happens:
- Playing Bard: When you play Bard, Mercurial, his ability triggers: "When you play me, if you paid the additional cost, move any number of your units to an open battlefield."
- Vex's Trigger: Vex, Apathetic's ability also triggers: "When an opponent plays a unit while I'm at a battlefield, [Stun] it. They can't move it this turn."
- Interaction:
- If you play Bard to the battlefield where Vex is present, Vex's ability will [Stun] Bard, and the restriction "They [you] can't move it this turn" will apply to Bard. Because this restriction is absolute, you will be unable to move Bard using his own ability (or any other effect) for the rest of the turn.
- If you play Bard to a different battlefield (not the one where Vex is present), Vex's ability does not trigger (as Vex's trigger condition requires the unit to be played while Vex is at a battlefield; however, if Vex is present at one of your battlefields, the trigger will apply to your unit regardless of where it is played).
- Correction/Refinement: Vex, Apathetic's ability triggers "When an opponent plays a unit while I'm at a battlefield." If Vex is on the board, he triggers when you play a unit. Bard will be stunned and unable to move.
Important Timing Note: Per RiftJudge FAQ #9475 and #10234, when you play a unit while Vex is on the board, Vex's ability is placed on the chain. This creates a "Closed State," meaning you cannot play [Action] spells to respond to the trigger. Because Vex's ability resolves and stuns Bard immediately, the restriction preventing you from moving him is active before you would have the chance to use Bard's "When you play me" movement effect.
Conclusion: If Vex, Apathetic is on the board, he will stun Bard as soon as he enters play, preventing you from using Bard's ability to move him to an open battlefield.