Does Unyielding Spirit prevent Elder Passive lethal damage?

No, Unyielding Spirit does not prevent the effect of Elder Dragon's passive ability, but it does prevent the damage that would otherwise mark a unit for that death.

Here is the breakdown of the interaction based on the official rules and clarifications:

1. Damage Prevention: Unyielding Spirit creates a "Prevent" action that replaces spell and ability damage. When Elder Dragon’s "When you play me" ability attempts to deal 1 damage to an enemy unit, if that unit is protected by Unyielding Spirit, the damage is prevented and is never considered to have been dealt (Rule 437.2.a).
2. Condition for Elder Dragon: Elder Dragon’s passive ability states, "Any amount of your damage is enough to kill enemy units." This ability requires that you have damage marked on that unit. If the damage is prevented by Unyielding Spirit, no damage is marked on the unit.
3. Outcome: Because no damage was dealt (and therefore no damage is marked by you), the condition for Elder Dragon's passive ability—having "your damage" marked on the unit—is not met. Therefore, the unit does not die from the passive effect.

In short: Unyielding Spirit prevents the damage itself, which stops the "marked damage" condition required for Elder Dragon's passive ability to trigger a kill.

Source: Rules FAQ Clarification 2026-04-29 (regarding Elder Dragon and Prevent effects).