If multiple Zhonya's Hourglasses are in play, do they all get used up when a friendly unit dies, or just one?

Ruling: Only one Zhonya's Hourglass gets used up when a friendly unit would die.

Sequence:

• The first Zhonya's Hourglass triggers and resolves
• After it resolves, the unit is no longer being killed
• The other Zhonya's Hourglass triggers fizzle because their trigger condition is no longer met