If I use Unyielding Spirit, then I use Fury of Blades, and I have an Elder Dragon in my base, and my opponent has two Ruined Rex in a battlefield, how would it resolve?

To resolve this interaction, we must follow the chain mechanics and the passive effect of Elder Dragon. Here is the step-by-step resolution:

The Chain

Since both Unyielding Spirit and Flurry of Blades are [Reaction] spells, they are added to the chain. Assuming you played them in the order you stated, the chain looks like this (LIFO - Last In, First Out):

1. Top: Flurry of Blades
2. Bottom: Unyielding Spirit

Sequence of Resolution

1. Flurry of Blades Resolves:

   • It deals 1 damage to all units at all battlefields.
   • Because you control Elder Dragon, its passive ability applies: "Any amount of your damage is enough to kill enemy units."
   • Since Flurry of Blades is your damage, the 1 damage dealt to the two Ruined Rex units is lethal.
   • Both Ruined Rex units die and are sent to the trash.
   • Deathknell triggers: Both Ruined Rex units have Deathknell abilities. These go onto the chain (above any remaining items) in the order of your opponent's choosing.

2. Chain Continues (Deathknell triggers resolve):

   • Your opponent resolves the Ruined Rex Deathknell triggers, dealing 4 damage to a target of their choice.

3. Unyielding Spirit Resolves:

   • Once the Deathknell triggers have finished resolving, the chain returns to Unyielding Spirit.
   • Unyielding Spirit resolves, granting its protection effect: "Prevent all spell and ability damage this turn."
   • Crucially: This protection applies to the remainder of the turn. If your units were targeted by the Ruined Rex Deathknell triggers, that damage was already dealt (as those triggers resolved before Unyielding Spirit finished resolving).

Important Nuances

• Order of Operations: Because Flurry of Blades resolves before Unyielding Spirit finishes resolving, the Ruined Rex units die and trigger their Deathknell effects before your units gain the protection from Unyielding Spirit.
• Elder Dragon's Passive: The passive effect applies because Flurry of Blades is damage controlled by you. Even though it only deals 1 damage, it satisfies the "lethal" requirement for enemy units due to Elder Dragon.
• Protection Timing: Unyielding Spirit only prevents damage after it resolves. Damage dealt by effects on the chain that resolve before it (like the Deathknell triggers) will still be dealt to your units.

This is my interpretation based on the standard chain resolution (LIFO) and the provided card abilities. No official FAQ exists for this specific combination.