Sorin Markovutility$7.41A very close functional match: it can set a player's life total to 10 immediately, just like Magister Sphinx. It also fits the same white-black-blue adjacent control shell in Commander even though it's a planeswalker.
Elesh Norn, Grand Cenobitefinisher$19.08Not the same effect, but it serves a similar high-end finisher role in W/B-based control decks by drastically swinging the board and ending games quickly. It is a comparable expensive threat with a dominating ETB-like impact on the game state.
Griselbrandfinisher$6.78A massive flying finisher in the same general high-mana threat slot. It doesn't set life totals, but it fills a similar 'drop one huge evasive creature to take over the game' role.
Dream Trawlerfinisher$2.51Another flying finisher in Esper colors that stabilizes the game and ends it through evasion and lifelink. It occupies a similar top-end control finisher slot.
Angel of Despairremoval$0.69A high-impact seven-mana ETB threat that answers a permanent when it comes down, similar to Magister Sphinx's immediate board impact. It is one of the closest old-school big control finishers in these colors.
Sphinx of the Steel WindfinisherBudget$0.21A comparable expensive flying artifact creature that stabilizes the board and is difficult to answer. It fills the same 'big artifact sphinx threat' niche in control decks.
Sunblast Angelremoval$0.59A white control finisher with a powerful ETB that can swing the game immediately. While it doesn't change life totals, it similarly punishes opponents when you resolve a big creature at the top of the curve.
Ashen RiderremovalBudget$0.33A premium ETB answer creature that exile-removes a permanent on entry or death. It plays the same role of a huge threat that also provides immediate value when cast.