You might want some hand attack like Thoughtsieze or Duress against combo. Ethersworn Canonist is good against storm. Kataki, War's Wage is good against affinity. Leyline of Sanctity is good against burn. Ghost Quarter or Tectonic Edge is good against tron. Stony Silence can also help against pod. (These being some of the more popular decks I see, but sideboard is dependent on local meta)

All of these are fabulous suggestions. I hadn't even begun to think up a sideboard.

Thank you very much.

I would also recommend Relic of Progenitus for sideboard. Also, and this might move this deck away from where you want it to go, but Xathrid Necromancer can combo very nicely with Athreos, God of Passage. Just a thought. Also, replace your Caves of Koilos with more Godless Shrines. +1.

NageekXLII Thanks for the suggestions. Relic of Progenitus isn't relevant in my meta. I never see delver deck builds so it probably wouldn't see any game time. Xathrid Necromancer on the other hand synergizes well with Athreos, God of Passage, Butcher of the Horde and Bloodsoaked Champion. He has been added. Thank you very much for the suggestions. :)

I'll start with something I suggest to everyone who makes a sac deck in Modern: Grave Pact. It gives you ridiculous board control against creature-based decks, especially paired with cards like Death Cultist and Viscera Seer, who can be sac'd for free and at any time. Speaking of Viscera Seer, what's it doing on your maybeboard? It's a free scry at worst, a great sac engine at best, and it's a one-drop! Definitely a mainboard choice, IMO.

Here are some other suggestions, based on my own experience with suicide black:

• Festering Goblin - great sac fodder early on. Can quickly get you rid of Delver of Secrets  ^Flip or Snapcaster Mage before they start getting feisty.

• Plagued Rusalka - when playing suicide black, you can't just wait for your creatures to get killed. You gotta have a sac engine. Cartel Aristocrat and Viscera Seer are the best, but Rusalka is almost as good, and it's pretty handy as spot removal, too.

• Vile Rebirth - A smaller Immortal Servitude that can target an opponent's graveyard. Killing Wave makes them sac their stuff too, so get something good out of it.

• Quest for the Gravelord - why not? It's not like you'll take longer than two turns to trigger this.

• Tragic Slip - I don't care if your dude is indestructible. I don't care if it survives Lightning Bolt or even Dismember. When I play this, it better be goddamn Emrakul or it's going down.

• Altar's Reap - good card draw for 2 mana, and the additional cost is irrelevant when you play suicide anyway.

• Bojuka Bog, Tormod's Crypt (sideboard) - when your opponents are sending as many cards to the graveyard as you are, you need some sort of graveyard hate to keep their recursion in check. Then again, nowadays there are so many goddamned Delver decks in Modern everyone should run graveyard hate, period.

• Path to Exile, Erase (sideboard) - Since you're playing Orzhov, might as well make use of the best white removal in the game.

And... I'm not sure I agree with Diabolic Tutor. Your deck has enough synergy that you'll rarely be in desperate need of a tutor, especially for such a high mana cost. Even Liliana Vess seems like a better choice. But that's up to you.

All in all, great job putting up that deck! I'd be delightfully annoyed if I played against it, that's for sure. +1 from me!

square711 Thanks for all the great input. After heaps of playtesting I found the Viscera Seer to be invaluable. I have no idea why I wasn't playing that one to begin with. Grave Pact was something I looked at earlier but decided to passed on. I don't remember why; however, after looking again I realized just how great it works with Athreos. Almost guarantees I get my saced Critter back to my hand. Out of everything else you mentioned the only thing I felt would make a difference was a couple of Altar's Reap. I'm always a fan of draw power.

Thanks again and I hope you had a Super Merry Christmas.