Greetings fellow Warlocks,
Here's a simple yet effective Demonlock.
Its a midrange-late'ish approach with lots of demon synergy.
A few very interesting combos ;
Soul Infusion + Doubling Imp = 2 4/4 for 4 mana
Anything that does self damage to power up Nethersoul Buster (Crystallizer , Flame Imp, Vulgar Homunculus and of course, your Hero Power)
Ectomancy, as all of your minions are demons anyway, so why not double up your board and their buff ..!
And Void Analyst, because why not.
... the rest doesn't really need any introduction or clarification : your typical OP warlock stuff.
Enjoy, and let me know in the comment how it works for you
PS: Here's a copy of the Demonology tomb, for reference.