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

Ruling: No, you cannot chain to The Boss's replacement effect. It is a replacement effect (not a triggered ability), which does not use the chain.

Sequence:

• The replacement effect intercedes during the execution of the kill effect
• It alters the execution directly without going on the chain
• No priority window opens for reactions

Nuances:

• The Boss received errata changing the ability from "When" to "If" with "instead" language, making it clearly a replacement effect
• The old version with "When" wording caused confusion because earlier rules stated replacement abilities could be triggered abilities
• Current rules removed that language, so replacement effects are only passive abilities and never use the chain
• Any ability type that uses the chain has an explicit rule stating so (e.g., "Activated Abilities use the chain")