Elder Dragon (Alternate Art)

While you control Elder Dragon on the board, 1 combat damage from your units is lethal to an enemy unit regardless of its might, unless another effect changes how much must be assigned or prevents that damage. Elder Dragon changes the lethal-damage value for enemy units with its controller's damage, and the smallest valid amount of damage is 1.^{142.3.b142.4.c418.1.e}

You must assign lethal damage to one unit before assigning damage to another, and you cannot assign more than the lethal minimum while another unit remains.^{467.2.c.3467.2.c.4} With Elder Dragon, that normally means assigning exactly 1 damage to an enemy unit before moving to the next. Passive abilities and replacement effects, including prevention, are considered during assignment and can change this minimum.^{467.2.c.4.a467.2.c.5}

Damage still marked by Elder Dragon's controller becomes lethal to enemy units when Elder Dragon enters, and opponents cannot react after it enters before the resulting cleanup handles that lethal damage. Elder Dragon's "your damage" means damage marked by its controller.^142.3.b Its first ability is a passive statement that changes the lethal-damage value for those enemy units while Elder Dragon is on the board.^{142.4.c364.2365.1}

Finalizing a unit does not pass priority, and the unit resolves immediately.^337.1.a337.2 After Elder Dragon enters, a cleanup happens without passing or awarding priority, and every unit with lethal damage is killed unless that death is replaced.^{319.6320.1323.4370.1370.1.c} Elder Dragon's separate "When you play me" ability is a triggered play effect, but it does not create a response window before this lethal-damage check.^{383.4.a383.4.a.2}

Yes, if you play Elder Dragon while an enemy unit at a battlefield has your damage marked on it and that unit's controller controls Zhonya's Hourglass in their base: Zhonya's Hourglass replaces that unit's death and recalls it before you choose targets for Elder Dragon's play effect, so you can choose the recalled unit in its controller's base. If another valid enemy unit remains at a battlefield, you can choose that unit as well because it is at a different location.

1. Elder Dragon enters, and its passive makes your marked damage lethal to the damaged enemy unit.^142.4.c Its play effect is then added to the chain as a pending item.^383.4.a.2

2. Both Elder Dragon entering and its play effect being added require cleanup before play continues.^319.6319.3 All outstanding cleanups finish before the pending effect can begin finalization.^320334.2

3. During cleanup, the damaged unit would be killed.^323.4 Zhonya's Hourglass replaces that death: the Hourglass is killed instead, and the unit is healed, exhausted, and recalled to its controller's base rather than killed.^370.1.a.1

4. Killing Zhonya's Hourglass sends it from the board to the trash, making another cleanup outstanding after the current one.^{429.1319.6322}

5. After all cleanups finish, you choose targets while finalizing the play effect.^{334.2337.1191.5.c403.2} The recalled unit is now in base, and any valid enemy unit that remains at a battlefield is at a different location, so both can be chosen.^{146.1198.1355.11.a}

When the effect resolves, it deals 1 to each target it affects, marking damage on each unit separately.^{418.1.b418.1.c} If Elder Dragon remains on the board, its passive makes that damage lethal, so each affected unit would be killed during the cleanup after the ability leaves the chain.^{142.3.b142.4.c365.1407.5319.5323.4}

Yes. The words "up to" allow Elder Dragon's controller to choose zero targets.^355.13 Because the ability does not begin with "you may," its controller cannot decline to perform it, so the play effect finalizes even when no enemy units remain.^{383.3.a403.4407.2}

After Elder Dragon enters, its play effect is added to the chain before the resulting cleanup.^{319.6334.2.a383.4.a.2} Elder Dragon's passive ability makes its controller's marked damage on enemy units lethal, so that cleanup may kill an opponent's last units.^142.4.c323.4 The pending play effect remains on the chain throughout the cleanup, so the opponent retains control of their empty battlefields and their [Hidden] cards remain there.^{320323.5323.6.a330331.1} After the play effect finalizes with zero targets, the opponent may play an eligible card from face down as a [Reaction] before the effect resolves.^407.4 When the effect resolves with no targets, it simply does nothing.