This is the Savjz Karazhan Medivh Control Mage Deck he was playing on ladder. Medivh, the Guardian has been working out in a lot of Mage decks, the synergy with high-value spells is pretty insane sometimes.
Control Mage, or Grinder Mage as it’s also been called hasn’t been a realistic deck to ladder with for some time. This card, along with Medivh’s Valet has changed the deck a lot. You now have the strong late game to compete with other control decks. You have Yogg, plus Alexstraza as win conditions, as well as overwhelming opponents on the board and using clears like Flamestike for tempo.