A spell on the Chain gives the other player Priority: the chance to answer before it happens. When both of you pass in a row, the newest item resolves.
While anything is on the Chain, only Reaction[Reaction] cards can be played. Reaction[Reaction] includes everything Action[Action] allows, and adds this.
Your Reaction goes on top, so it happens first: make your unit bigger, move it, or cancel the spell.
After you answer, Priority goes back to them: answers can pile up, and the last one in happens first.
If you won't answer, pass.
Your runes' tap and recycle abilities are Reactions too, so you can pay for a Reaction even on the opponent's turn, as long as you kept a rune ready.
Quick check
Your opponent's spell is on the Chain. You hold Hextech Ray (Action[Action]) and Discipline (Reaction[Reaction]). Which can you play now?
Predict
You attack with two units. Jinx holds Gust (return a unit with 3 Might or less). Do you play Discipline (+2) right away, or wait?
Make your guess, then play the drill to find out.
Discipline: Reaction[Reaction]. +2 Might this turn, and draw 1.
Hextech Ray: only an Action[Action]. It can't be played while the Chain is open.
Everything Action[Action] allows, plus playing the card while something is on the Chain.
From the Core Rules (2026-10-09): 813 — Reaction · 327 — Chains · 312 — Priority · 337 — units and Add abilities resolve at once · 164 — rune abilities are Reaction[Reaction]s
Try it on the table
3 drills
1Answer on the Chain
It's Darius's turn. He's about to cast Hextech Ray (deal 3) at your Stellacorn Herder (3 Might) at A. You hold Discipline (2 Energy, Reaction[Reaction]: give a unit +2 Might this turn, draw 1). Press Next to see the Ray go on the Chain. Before it resolves, play Discipline on the Herder. Yours resolves first, so the Herder is 5 when the 3 damage lands.
Need a hint?
Press Next until Hextech Ray is on the Chain and you have Priority. Then click Discipline and target your Stellacorn Herder.
You're on 7, B is scored, and Leona's Adaptatron (3) holds A. Cast Hextech Ray (deal 3) at it. She'll answer with Discipline (+2 Might). After her answer, Priority comes back to you: Defy (Reaction[Reaction]: counter a spell) goes on top and resolves first.
Need a hint?
Ray the Adaptatron. When Discipline appears on the Chain, play Defy.
You're on 7, B is scored, and Jinx's Shadowblade Lurker (5) holds A. Bilgewater Tracker (3) and Kinkou Initiate (3) beat it together, but Jinx holds Gust (Reaction[Reaction]: return a unit at a battlefield with 3 Might or less to its owner's hand). You have one Discipline (+2 Might this turn). Attack with both, pass, and spend it on whichever unit she Gusts, in response.
Need a hint?
Attack with both and pass. When she Gusts a unit, Discipline that same unit.
