Where do you play Zhonya's Hourglass - battlefield or base, and how does its activation work?

Ruling: You can choose to play Zhonya's Hourglass either to your base (open and ready) or hide it at a battlefield. When played to your base, it functions as a passive replacement effect that automatically triggers when the next friendly unit dies, regardless of whose turn it is.

Sequence:

• If played to base: Goes down ready, opponent cannot react to it being played, remains in base looking for the next friendly unit death
• If played hidden to battlefield as a reaction: Goes to the battlefield when activated, then returns to base during cleanup

Nuances:

• Zhonya's uses a replacement effect, not an activated ability - it automatically saves the next unit it sees die
• Playing it hidden to battlefield offers a cheaper payment method and surprise factor
• When Zhonya's is visible in base, opponents can strategically kill a small unit first to waste its effect