If Zhonya's Hourglass is hidden on a battlefield, and I charm the only unit there to another battlefield. Does it still able to save that creature from dying on the other battlefield

Question

Accepted Answer

Yes. If Zhonya's Hourglass is a hidden permanent on Battlefield A, and you use Charm to move the only unit from that battlefield to Battlefield B, Zhonya's Hourglass can still replace that unit’s death on Battlefield B provided the Hourglass is still in play and its replacement effect applies to that unit.

Yes. A hidden permanent’s abilities and replacement effects function normally while it’s on the battlefield, so Zhonya's Hourglass can replace a friendly unit’s death even if the unit has been moved to a different battlefield, unless the replacement effect specifically requires the Hourglass to be in a different zone or specifies a location restriction.

Zhonya's Hourglass is a hidden permanent on Battlefield A; hidden permanents are still on that battlefield and their abilities function normally while there [811]. You cast Charm and move the only unit from Battlefield A to Battlefield B; the unit remains the same game object on the board (a battlefield-to-battlefield move). If that moved unit would die on Battlefield B, the game checks for replacement effects that can apply to that death; a replacement effect on Zhonya's Hourglass that cares about "a friendly unit would die" is a qualifying replacement effect and is evaluated [370]. Because Zhonya's Hourglass is still in play and its replacement effect isn’t limited to the battlefield it occupies, it may be applied to replace the unit’s death (this is consistent with the Zhonya’s example in the cost-paying rules) [357]. If the Hourglass is removed or if its replacement effect explicitly required it to be in a different zone or to target a unit at the same battlefield, it would not be able to replace the death.

If the unit is moved to a non-board zone (hand, trash, banished) before the death event, it is a different object and Zhonya's Hourglass cannot replace that death [359]. If Zhonya's Hourglass's replacement effect explicitly required the unit to be at the same battlefield as the Hourglass (or required the Hourglass to be in some other specific zone), that restriction could prevent the replacement from applying [370]. If the Hourglass is removed, destroyed, or otherwise leaves the battlefield before the death event, it cannot replace the death. If another replacement effect or ordering of simultaneous replacement choices prevents the Hourglass from applying, standard replacement-effect resolution rules determine the outcome [370].

Hidden permanents placed on a battlefield still have their abilities and instructions function normally while they are there, including replacement effects [811]. Charm moving a unit from one battlefield to another does not by itself remove the unit from the board, so the unit remains the same game object on the board when it later would die. When a unit would die, the game checks replacement effects that can apply; a replacement effect on Zhonya's Hourglass that reads “If a friendly unit would die, kill this instead…” is such an effect and can be evaluated and applied while the Hourglass is in play [370]. The rules even use Zhonya's Hourglass as an explicit example of a replacement effect replacing a death that occurred while paying a cost, showing the Hourglass can replace a friendly unit’s death while the Hourglass is on the battlefield [357]. The only times this would fail are if the unit left to a non-board zone (in which case it’s a different object and replacement relationships to the original object are severed) or if the Hourglass’s replacement effect specifically required it to be in a different zone or at the same battlefield as the dying unit [359] [370].