Sigil of the Storm when I conquer, can I recycle Seal of Unity or does it need to be a Rune?

No. You must recycle an actual rune card — a Rune from your rune deck or runes on your board. Seal of Unity is a Gear, not a Rune card type.

Sigil of the Storm says "recycle one of your runes" — that's a specific game action requiring a card of the Rune type. Exhausting a Seal generates Power, but it does not perform the action of recycling a rune (FAQ #6663, #8821, #10631). Same principle applies to recycling a Gold Gear token or any other non-Rune object (FAQ #8846).

If you have no runes available, the trigger resolves doing nothing and you still conquer (FAQ #6863). But as long as you have runes, you must recycle one — not a Seal.