If I try to empower my Steel Paws for 7 energy, and my opponent responds by returning the Steel Paws to my hand, does my Nasus legend still trigger to give me 2 runes back?

Yes — Nasus still triggers. The Empower ability was fully activated (cost paid, placed on the Chain) and it resolved, even if its effect did nothing because Steel Paws left play.

Here's the sequence:

1. You activate Empower [7] on Steel Paws — pay 7 energy, ability goes on the Chain.
2. Your opponent responds (Reaction), returning Steel Paws to your hand. That resolves first.
3. The Empower ability then resolves. It tries to empower "me" (Steel Paws), but Steel Paws is no longer on the board — the effect does nothing. However, the ability still resolved.

Under the clarified rules, "play" in a trigger condition means the card/ability has resolved (Patch Notes 2026-07-17). Activated abilities on the Chain resolve independently of their source — they don't "fizzle" just because the source left (FAQ #4917, #3929). Since Empower resolved, the condition for Nasus ("when you play an activated ability with Energy cost [7] or more") is satisfied.

So you may exhaust Nasus to ready up to 2 runes.