What happens if I kill a unit with Smite and that unit has a Deathknell effect?

When you play Smite to kill a unit with a Deathknell effect, the Deathknell effect will not trigger.

Here is the breakdown of why this happens based on the rules:

1. Replacement Effect: Smite contains the text: "If it would die this turn, banish it instead." This is a replacement effect (Rule 369.1).
2. Deathknell Requirement: According to Rule 808.1.d, the trigger for Deathknell is the permanent being Killed and sent to the Trash.
3. Replacement of Death: Rule 808.1.d.1 explicitly states: "If the Permanent with the effect is not sent to the Trash, for example because its 'killed' event was replaced with a recall, the Deathknell will not occur."
4. Outcome: Because Smite replaces the "die" event with a "banish" event, the unit is never sent to the trash. Consequently, the Deathknell condition is never met, and the ability does not trigger.

This is consistent with how other replacement effects (like Zhonya's Hourglass) interact with Deathknell, as confirmed by RiftJudge FAQ #2596.