Am I able to decline to pay the Deflect cost, fizzling the Deathknell ability of the Ruined Rex when it dies?

Yes, you can decline to pay the Deflect cost, and the Deathknell ability will be removed from the Chain without resolving.

Here's how it works step by step:

1. Ruined Rex dies → its Deathknell triggers automatically (you cannot opt out of the trigger — it's mandatory per Rule 808.1).
2. When the Deathknell trigger tries to finalize on the Chain, it must choose a target enemy unit. If that unit has Deflect, the ability incurs a mandatory additional cost of [A] (1 Power).
3. However, Rule 404.2 gives a special exception for triggered abilities: "players may decline to pay for Triggered Abilities that have incurred a cost. If they do, the ability will cease being a Pending Item and be removed from the Chain."
4. If you decline, the Deathknell never becomes a Chain Item and does nothing. No damage is dealt.

This is confirmed by FAQ #5102 (Ahri Inquisitive's triggered ability + Deflect) — the same principle applies here. The Patch Notes from 2025-10-24 also note this is a specific design choice: "Players can now refuse Deflect costs incurred by triggered abilities."

One nuance: this is not "fizzling" — Riftbound doesn't use that term. The ability simply fails to finalize and is removed before it ever enters the Chain. Also, this only applies to triggered abilities (like Deathknell). For spells, Deflect remains a mandatory cost you cannot decline (FAQ #9431).