Mulligans
General Mulligans
These are all solid cards to start with in your hand - especially Knife Juggler and Possessed Villager. Early on it's a matter of trading minions for minions and Imp Gang Boss helps keep a presence on the board whilst buffing the Darkshire Councilman with every Imp spawned. Knife Juggler and Disciple of C'Thun can remove early game minions.
Possessed Villager is of only limited use if you don't have either the Councilman or the Juggler in your hand to start though - in which case you can probably mulligan it out and hope for something with more immediate impact.
Essentially this is a deck based around synergy between Forbidden Ritual, Imp Gang Boss, Possessed Villager, Darkshire Councilman, Knife Juggler, Usher of Souls and of course, C'Thun.
In a way it’s almost an anti-C’Thun deck; early on the aim is to try and control the board with disposable minions buffing the Darkshire Councilman and C’Thun (with Usher of Souls).
If your opponent drops something big (like C’Thun) late game you may have the Siphon Soul option or, if they’ve got several large minions out (e.g. Twin Emperor Vek'lor and friends), DOOM! – which will nicely replenish your hand and hopefully, if you’ve not drawn it already, may pull C’Thun.
The tech cards are just Faceless Manipulator (useful against Druid) and Sylvanas Windrunner (which can delay an opponent’s C’Thun drop at least) – this is a seriously minion-heavy deck, so the more minions the better… and they can copy/steal your opponents best minions while they’re at it.