Chain and Priority

Can I react to units being played?

You cannot react to another player's unit before it enters the board. Finalizing the unit does not pass priority; the unit instead resolves immediately and enters the board.^{[337.1.a][337.2][359.2.c]}

The unit's controller has one exception: while paying the unit's costs, they may activate an ability with Reaction that uses Add.^[357.1.a] That ability finalizes and resolves immediately during payment.^[444.2.c]

After the unit enters, its play effect and any other fulfilled triggered abilities are put on the chain.^{[383.2.c][383.3][383.4.a.2]} You may react once those abilities finalize and priority is granted, but the unit is already on the board by then.^{[337.4][338.1.a][338.1.a.2]}

In what order are multiple pending chain items finalized and resolved?

Pending items undergo finalization from oldest to newest, finalized items still waiting on the chain resolve from newest to oldest, and units, gear, and Add abilities resolve as soon as they finalize.

• Pending items: Take them through finalization in the order they were added, from oldest to newest, until each item finalizes or leaves the chain.^{[337.1][337.1.b]}
• Finalized items: Among those that remain on the chain, the newest resolves first.^[340.1]
• Immediate resolution: Units, gear, and abilities that use Add resolve when they finalize, even if other pending items still need to finalize.^[337.2]

Can I play a reaction after one, but not all, items on the chain have resolved?

Yes. If another finalized item remains after one item resolves, the newest remaining item's controller receives priority before it resolves once any pending items have finalized or left the chain.^{[337.1][340.3][340.4]} That player may use this priority to play a legally timed Reaction.^{[338.1.a][338.1.a.2]} The chain therefore does not have to resolve in its entirety once resolution begins; a new item can be added between two resolutions.

Outside a showdown, do I lose control of a battlefield as soon as my last unit leaves it?

If a chain is still in progress or combat is ongoing at that battlefield, your last unit leaving does not make you lose control during the resulting cleanup.

Battlefield control is checked during cleanup. An item remaining on the chain keeps the empty battlefield under your control.^{[323.6][330][309.1]} An ongoing combat at that battlefield also prevents control of that battlefield from changing.^[190.4.b] Removing the chain's final item causes a later cleanup to check again; if the battlefield is still empty and no combat is ongoing there, you lose control then.^{[190.4.c][319.5][309.2]}