How does Singularity resolve? I have a Scuttle Crab, a Stellacorn Herder and a Zhonya's Hourglass. Does the player who uses Singularity get to choose the order in which units die?

Singularity resolves as a single event that deals 6 damage to each of the chosen targets simultaneously. Because it is a single spell resolving, there is no "order" to the damage being dealt; it happens all at once.

Here is the breakdown of how your situation resolves:

1. Singularity Resolution

• You choose up to two units to be dealt 6 damage. Since Singularity says "Deal 6 to each of up to two units," you could target your Scuttle Crab and your Stellacorn Herder (or only one of them, or one of yours and one enemy unit).
• The damage is dealt to all targets simultaneously.

2. Lethal Damage and Replacement Effects

• Once damage is dealt, the game checks for "death" (units with damage equal to or greater than their Might being sent to the trash).
• Because your Scuttle Crab (0 Might) and Stellacorn Herder (3 Might) are both dealt 6 damage, they are both considered to have "died" at the same time.
• Zhonya's Hourglass is a replacement effect. Per Rule 373, when multiple units you control die simultaneously and you control a Zhonya’s Hourglass, you (the controller of the Hourglass) choose which unit’s death event to apply the Hourglass to first.

3. Execution of Zhonya's

• You choose one unit to save: If you choose the Scuttle Crab, the Hourglass is destroyed, and the Scuttle Crab is healed, exhausted, and recalled to your base.
• The other unit dies: Since the Hourglass is a replacement effect and is sent to the trash as part of its cost to save that first unit, it is no longer available to save the second unit. Therefore, the second unit (the one you did not choose to save) proceeds to the trash and is killed.

Summary

• The player who uses Singularity does NOT choose the order of death. The spell resolves all at once.
• You (the owner of the Zhonya's Hourglass) choose which of your units is saved by the Hourglass because the death event happens simultaneously for both units.
• The other unit will die normally, as the Hourglass is consumed by saving the first unit.

Note: This is my interpretation based on Rule 373 regarding simultaneous events and the specific function of replacement effects like Zhonya's Hourglass. No official FAQ exists for this exact three-card combo, but this is the standard mechanical interaction for simultaneous deaths.