This is a control Priest with N'Zoth, the Corruptor, Zephrys the Great and quest.

The goal of this deck is to control the early-mid game, and then drop N'zoth as a win condition. There's also quest, which after completed (with the help of cards such as Khartut Defender or [Hearthstone Card (Deranged Doctor) Not Found]) gives a lot of value in the late game.

Mulligan:

Northshire Cleric, Injured Tol'vir, Injured Blademaster, Acolyte of Pain

Against aggro it's good to keep Mass Hysteria or Holy Ripple.

Seance is very good when used on Zephrys the Great or N'zoth (if it survives), but copying another deathrattle minion (Sylvanas, Khartut) is good as well.

Archivist Elysiana helps in longer matches (especially against Control Warrior). If you manage to cast Seance on her, you get double value.