Does the Harrowing banish a unit before playing it? Does it empowers Zed legend?

No — The Harrowing never banishes anything. Its text is just "Play a unit from your trash, ignoring its Energy cost," so the chosen unit moves trash → chain → board as a new game object; banishment is only involved when an effect explicitly says "banish" (Arcane Shift, Portal Rescue).

Two things that follow from that: since it's a play and not a banish, it's not a "banished" event for any trigger, and it does target the card in your trash when the spell finalizes, so an opponent can react with something like Disposal Order to remove it first (FAQ #12258, Rule 359.3.e).

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