Learn Riftbound
Every Riftbound card answers two questions: is there a showdown going on, and is there a chain open? Pick a card, pick whose turn it is, and the board lights up wherever it can be played. Tap any square for the reason, straight from the Core Rules.
Playable any time you have Priority.
Yes — Wind Wall, neutral closed, your turn. Reaction, so it can go on top of a chain that's already open — it resolves first.
Playing a card puts it on the chain. While anything is on the chain, only Reactions can join it. Priority goes around the table; when everyone passes in a row, the newest item resolves, then the next.
Try it: play Falling Star at their unit, pass, have them answer with Discipline, then counter that with Wind Wall. Then start over and let Discipline resolve instead — the unit lives.
Nothing on the chain. Play Falling Star to start one.
Moving your units onto a battlefield where an opponent has units starts combat, and combat opens with a showdown. The attacker gets Focus first. The player with Focus plays one Action or Reaction (its chain resolves, then Focus passes) or passes. Two passes in a row end the showdown, and the units fight.
Falling Star won't work here — it has no timing keyword. Cleave will.
Timing keywords only give permission — they never change what a card does. Everything else about the card stays the same, including where a unit can go.
Your turn, when nothing else is going on.
Your turn, plus any showdown — yours or theirs.
Any time you have Priority — including on top of a chain.
Reaction timing, then it attaches itself.
Reaction timing — when played to a battlefield where you already have units.
Hide it on your turn; play it from facedown from your next turn on, at Reaction speed, for free.