Council of Echoes

Six mana for an evasive body with a tempo bounce stapled on would be soft in a vacuum, so the Descend 4 gate makes you earn the elastic half by having already spent resources: fetched lands, dead creatures, cycled cards, milled fodder, whatever raises the permanent-card count in your graveyard to four before the Spirit lands. Miss the count, and the intervening "if" clause switches the trigger off entirely; you keep a 4/4 flyer and nothing more, which is the honest read on what the card asks of the deck built around it. Meet it, and the "up to one" wording earns its keep: it lets you decline the bounce rather than being forced to return your own permanent when the opponent has no target worth touching. Descend does the structural work that delve and self-mill payoffs do elsewhere, converting a graveyard you were filling anyway into a conditional swing. The target line is deliberately wide (any nonland permanent other than itself), so it can reset an opposing threat, reload one of your own creatures' enters-the-battlefield triggers, or clear a blocker before the alpha strike. Because the bounce triggers when the creature enters, the window is not reactive: you cannot hold it back to answer removal on the stack, so the play is choosing the turn where the yard is stocked and the board most rewards a free bounce, rather than jamming it on curve.