Can you chain to Sett the Boss's replacement effect that prevents a unit from being killed?

Question

Accepted Answer

Yes. When a buffed unit you control would die, Sett - The Boss’s optional replacement ability can be placed on the chain. During finalization you may pay the rainbow rune, exhaust Sett - The Boss, and spend the unit's buff as the costs to have the ability resolve and recall the unit instead of it being killed.

You may place Sett - The Boss's “When a buffed unit you control would die, you may pay :rb_rune_rainbow: and exhaust me to spend its buff and recall it exhausted instead” ability on the chain as an optional triggered/replacement effect. If you choose to put it on the chain, you must pay its costs during finalization (pay the rune, exhaust Sett - The Boss, and spend the buff) for the effect to finalize and replace the kill with a recall. If you decline to place it on the chain or cannot pay/meet its costs during finalization, the replacement does not occur and the unit will be killed normally. If the death is replaced by the recall, any Deathknell on that permanent will not trigger. [FAQ:unleashed#2] [355.10.c.1] [357.2.a] [808.1.d.1]

A buffed unit you control would die; the game recognizes the would-die event and checks for replacement/triggered effects that respond to that event. You may choose to place Sett - The Boss’s ability on the chain as a pending triggered/replacement item (the choice to place it on the chain is made when the trigger condition is fulfilled) [FAQ:unleashed#2]. If you place it on the chain, the ability is finalized by making choices and paying costs during finalization: pay :rb_rune_rainbow:, exhaust Sett - The Boss, and spend the unit's buff (the spend is a cost within the instruction) [355.10.c.1] [FAQ:unleashed#2]. If you successfully pay those costs during finalization, the ability becomes a finalized chain item and will resolve, replacing the kill with the recall (the unit is sent to base exhausted instead of being killed). If you decline or fail to pay/meet costs, the replacement does not occur and the unit is killed normally. If the kill is replaced by the recall, any Deathknell on that permanent will not trigger because the permanent was not sent to the Trash [808.1.d.1].

If the unit changes to a non-board zone (e.g., is removed from the board) before finalization finishes, it may no longer be the same object and relevant checks can return null or make instructions impossible to follow [359.3.e.4]. If the replacement is being considered as part of paying an additional cost (for example, a cost that requires killing a unit), a replacement that changes the kill to a recall still counts as having paid that cost per the example in [357.2.a]. If you cannot pay the rainbow rune or cannot exhaust Sett - The Boss or cannot spend the unit's buff during finalization, you cannot finalize the replacement effect and the unit will die normally [355.10.c.1] [357.2.a]. If the unit is not actually 'buffed' (i.e., there's no buff to spend), the condition for Sett - The Boss's ability is not met and you cannot place it on the chain.

Sett - The Boss’s text reads like an optional triggered/replacement ability that applies "When a buffed unit you control would die…" That kind of ability is placed on the chain as a pending triggered item when the condition is met; the controller then finalizes it by making the choices and paying any costs required by the instruction sequence [FAQ:unleashed#2]. The parts of the ability that look like costs (paying the rainbow rune, exhausting Sett - The Boss, and spending the unit’s buff) are costs within the instruction and therefore become costs that must be paid during finalization, not separate effects of the ability [355.10.c.1]. If you place the ability on the chain and successfully pay those costs during finalization, the effect becomes a finalized chain item and will resolve, replacing the kill with a recall that sends the unit to base exhausted instead of being killed. If you decline to put the ability on the chain or cannot pay/fulfill its costs during finalization, the replacement does not occur and the unit is killed normally. Note that if the kill is replaced by the recall, Deathknell will not trigger because the permanent was not sent to the Trash [808.1.d.1]. Also be aware of timing and object identity: if the unit is moved to a non-board zone (or otherwise ceases to be the same object) before finalization/resolution, checks can return null or make instructions impossible to follow [359.3.e.4]; and if the replacement is involved while paying an additional cost, a replacement that prevents a death still counts as having paid that cost per the example in the rules [357.2.a]. In short: you can "chain to" (place on the chain) Sett - The Boss’s replacement ability; whether it ultimately replaces the kill depends on paying its costs during finalization and on the unit still being the same legal object when those checks occur [FAQ:unleashed#2] [355.10.c.1] [357.2.a] [359] [808.1.d.1].