Does Singularity kill a unit with a Guardian Angel?

To answer your question: No, Singularity does not kill a unit equipped with Guardian Angel.

Here is the breakdown of why this interaction occurs, addressing the rules and mechanics correctly:

1. The Interaction

• Singularity Resolution: When Singularity resolves, it deals 6 damage to the target unit.
• Lethal Damage Check: In Riftbound, lethal damage is checked after the spell finishes resolving. If that damage is lethal, the unit would normally be "Killed" (sent to the trash).
• Replacement Effect: Guardian Angel has a replacement effect: "If I would die, kill Guardian Angel instead. Heal me, exhaust me, and recall me." Because Guardian Angel replaces the "death" event, the unit is healed and recalled to your base rather than being sent to the trash.

2. Addressing the "Kill Action" and Attribution

You are correct to note that attribution rules are specific.

• The Kill Action does not occur: Because Guardian Angel is a replacement effect, the event of the unit being "Killed" is replaced by the event of the Guardian Angel being destroyed and the unit being recalled. Since the unit never enters the trash, no "Kill" action takes place for that unit.
• Attribution Rule 428.5.c: This rule explains how to attribute a Kill action when one occurs due to Cleanup. Since the unit is protected by Guardian Angel and does not die, this rule is not triggered. The unit survives, and the spell (Singularity) is not credited with a Kill for that unit.

3. Summary of Findings

• Guardian Angel is a Replacement Effect: It intercepts the game state change (moving to the trash) and substitutes it for a different outcome (recall to base).
• The Unit Survives: Because the death is replaced, the unit does not enter the trash, and therefore the criteria for being "Killed" (Rule 428.1: "Killing is the action of a Permanent going to the trash from the board") are not met.
• Guardian Angel is Destroyed: The gear itself is sent to the trash as the cost of the replacement effect, but the unit remains on the board/in the base.

This interpretation is consistent with how replacement effects function in Riftbound to prevent specific game events. No official FAQ exists for this exact interaction, but it follows the mechanical definition of replacement effects and the "Kill" definition provided in Rule 428.