If I have an "inferna" at a battlefield with a hidden "here to help" and an opponent uses a "hextech ray" to kill my "inferna" and I react playing a "brynhir thundersong" does that cause the original hextech ray to fizzle out?

No, playing Brynhir Thundersong will not cause the original Hextech Ray to "fizzle."

Here is the breakdown of why this interaction functions this way:

1. Timing and Resolution

When your opponent plays Hextech Ray, it is placed on the chain as a finalized item. Because it is an [Action] spell, it creates a closed state on the chain, meaning only [Reaction] cards can be added in response.

When you play Brynhir Thundersong from your hand in response to the Hextech Ray, you are essentially playing a card (or reacting, if permitted by other game rules, but regardless of how you enter the chain, the chain order remains). The Brynhir Thundersong ability triggers and is added to the top of the chain.

According to the rules of the chain (Rule 327):

1. The chain resolves in "Last-In, First-Out" (LIFO) order.
2. The Brynhir Thundersong play ability resolves first. This applies its restriction ("opponents can't play cards this turn").
3. After the Brynhir Thundersong effect resolves and clears, the next item on the chain is the Hextech Ray.

2. Why it doesn't "fizzle"

A spell only "fizzles" (is ignored) if its targets become invalid before it resolves.

• Brynhir Thundersong has no effect that removes your Inferna from the board, changes its status, or alters the Hextech Ray's targeting requirements.
• The restriction imposed by Brynhir Thundersong ("opponents can't play cards") restricts future play actions; it does not retroactively undo or counter a spell that has already been finalized on the chain.

Since the Inferna remains a valid target at the battlefield when the Hextech Ray finally resolves, the spell will successfully execute its instruction and deal 3 damage to the Inferna.

Note: This is my interpretation based on the standard LIFO chain resolution rules. No official FAQ exists for this specific interaction.