Is Zhonya's Hourglass trigger optional, or must you use it when a unit dies?

Ruling: Zhonya's Hourglass is not a trigger but a mandatory replacement effect. You cannot choose to let the unit die to save the Hourglass for later.

Nuances:

• The only time you have a choice with Hourglass is if multiple units die simultaneously, in which case you choose which one to save.