*Probably abandoned idea.  Every Ob Nixilis deck looks exactly the same and Urborg/Crucible is just too expensive and without them the deck is pretty gimp.

Mono black EDH deck I've been struggling with for the longest time.  I must have broken it down 4 times now.  It went from Korlash to Kuro, then back to Korlash.  Then back to Kuro once I saw a nifty combo with Eternity Vessel.  And now to Ob Nixilis.

Here's some of the combos:
Ob Nixilis + Ghost Town (or even Thawing Glaciers)
You should see that coming.  It's basically guaranteed landfall.

Blood Speaker + Deathrender
Because where else are demons playable.  Oh woops, Blood Speaker just came back to my hand.

Blood Speaker + Reaper From the Abyss
If the Reaper is already in your hand...it's not all that, but maybe later that turn you can cast Reaper, return Blood Speaker back to my hand, get a Morbid trigger to boot.  It's a simple combo but still effective.

Kuro + Eternity Vessel
I'm trying to shoehorn lifegain into the deck, but you can search up Kuro with Blood Speaker and you want Landfall with Ob Nixilis anyway, so why not wipe out everything?  Who cares about life.