Batterskull and Phyrexian Obliterator if you can afford them.

http://tappedout.net/mtg-decks/grixis-obliteration/ non-budget version

This looks rather slow for a Modern deck. I'd lower your creature count to only include those that can survive and win you the game or early beaters like Delver. Creeping Tar Pit is an amazing land to run. I'd suggest cutting all of the creatures minus Delver to get some more removal, countermagic, and board wipes. You'd have 7 slots open, so maybe 2 Spell Pierce, 1 Spell Snare, 1 Creeping Tar Pit, and 2 Electrolyze / Young Pyromancer?

I'd be suggesting Batterskull and Damnation if it were in your budget as well.

I am also concerned that this might be on the slower side, assuming you want to take it to a competitive Modern event and expect to win a few rounds. A few of these cards, such as Cruel Ultimatum and Garza Zol, Plague Queen, really only work in Commander because of their mana costs. (Commander decks with Garza at the helm are freakin' sweet, by the way.)

If you're going budget Grixis, I'd say look into a creature base focused on Delver of Secrets  ^Flip, Guttersnipe, Vampire Nighthawk, Young Pyromancer, and Blood Artist. You are running Pyroclasm and Black Sun's Zenith, which will more often than not hurt as much as they help unless you have a Blood Artist on the board. Blood Artist is also really handy with Young Pyromancer. It basically says, "block them or not, these tokens are going to hurt you." Even after you've sideboarded in Illness in the Ranks, those tokens still give you value, even though they're dying the second they hit the battlefield, because they are triggering Blood Artist.

If it were me, I would run full playsets of each of those creatures, so I'd remove Garza, Grave Titan, Reaver, and Shimian altogether. All are great cards, but are either too high a cost or too conditional to merit a slot here.

I think that will speed up the deck considerably. Your instant and sorcery base is pretty solid, aside from Cruel Ultimatum, which is just too hard to pull off in competitive Modern.

Countersquall should probably be moved to the sideboard. The good news is, your sideboard has some pretty good stuff already, as well as some other things that should either be moved to the mainboard or just taken out altogether.

Illness in the Ranks is great sideboard tech against Kiki-Jiki, Mirror Breaker and Splinter Twin, so keep it there. That one spell can just completely destroy those decks.

I would actually just replace your mainboarded Terminate with the Dreadbore in your sideboard, since they cost the same, but one does more than the other, and if you find yourself playing against a creatureless deck, all that spot removal is going to be a thorn in your side.

I would take out Oona, and replace it with 2x Duress and 2x Despise if you aren't ready to drop the dough on Inquisition of Kozilek (though Inquisition is hands-down going to be your best option).

Think Twice is really not a sideboard card. It's either mainboarded or not included at all. Remember that your sideboard is there to help you respond to your opponent's deck, not just beef up your mainboard.

You've got graveyard and artifact hate with Rakdos Charm, but I still think a Grafdigger's Cage, or at least a Tormod's Crypt could really help against Scapeshift and storm decks.

Aside from that, my only suggestion is that Crumbling Necropolis works against you a bit if you are going to use checklands like Drowned Catacomb or Dragonskull Summit. You might be better off using Evolving Wilds or Terramorphic Expanse, since they are going to get you the lands you need to have those checklands come in untapped. Shock and fetch lands are of course better, but if you're going budget, that's the smarter way to use checklands. They also have the added benefit of removing two lands from your deck rather than one, and deck-thinning is the next best thing to extra draws.

Hey man, it's okay to play a slow control deck in Modern, you just need to stop your opponent from killing you until you can do your thing. It's easier said than done but certainly possible.

If you want to talk about specific card choices and all that, let me know! For starters though, I would suggest considering different creatures as win conditions because Lightning Reaver dies so easily and AEtherling can be really, really slow to win.

Cruel Ultimatum seems bad here unless your building around it and discard spells dont fit in very many decks in modern definitely not grixis control

I don't think you should be mainboarding 3 copies of Counterflux, unless there's a bunch of storm in your meta. If you want a 3-mana hard-counter, I'd go with Dissolve.

Also, you should be playing 4 Bolts. Always 4 Bolts.

The reasoning for Counterflux is it being uncounterable. Four bolts has ended up being too much, as it only really ever hits 1 or two creatures in any deck. Most everything else is 4+ toughness.