First off, I think there is promise in a Jund build but I don't think it will make the leap to being anything close to tier 1 until we see the other Scry lands in Born of the Gods.
That being said, a few thoughts:
1) There's really no need for Hero's Downfall in here as Dreadbore works sufficiently 95% of the time. Given that when an opponent plays a PW, they still hold priority and can activate it before you could kill it anyway, why not save the 1 mana and just do it Dreadbore style? (Mind you, I get that you can cast Downfall on an opponent's turn, saving your mana for your turn, but really, it's not THAT big of a deal at the moment).
2) I would actually drop a few of the guildates for just run of the mill lands. I know they you're trying to maximize the likelihood you hit all your colors as quickly as possible, but the fact guildgates come into play tapped will often leave you a turn behind even when you're hitting your land drops. The added ability of the Scry lands to let you sift through your top decks justifies the ETB tapped, but otherwise, they'll just slow you down more than you probably want. So to that end, a full playset of the shocks, perhaps 1-2 guildgates (1 Rakdos, 1 Golgari) and 4 Temple of Abandon should give you access to enough fixing that run of the mill Swamp, Mountain and Forest should get the job done.
3) I can see where you're going to want some number of Abrupt Decays in here. The number of non-creature permanents it can take out is significant and meaningful. Think Detention Sphere and Underworld Connections - in addition to dropping an opponent's Boros Reckoner and whatnot.
4) Given the high number of shocks you're running, I don't know that I would run Thoughtseize in the main board and if I did, no more than two. Odds are you're going to be siding them out a LOT against even moderately aggressive builds as they, combined with the shocks, end up doing half of the opponent's job for them. To that end, if you're really wanting to look at the opponent's deck/hand, I cannot recommend enough that you use Slaughter Games. Most people will say, wait, you should be contributing to the board with your four drops, and, while I agree most of the time, a single Slaughter Games will practically neuter both mono-blue and mono-black devotion decks, esper rich kid/superfiends/control, and a LOT of other strategies. In the worst case scenario, you name a card that you THINK they might have and get to look at their hand and entire deck - that's the kind of information you just cannot overlook in any type of competitive play. Additionally, Duress in the SB does a solid job against control and doesn't cost the life of Thoughtseize. So, the tl;dr version, maybe 2x seize in the main, the rest in the SB along with Duress and give a look at Slaughter Games in the main (even 1-2x could be useful) -- I know this is not a popular or widely felt sentiment, but I would argue that it can be huge if played correctly.
5) Anger of the Gods should be in the SB. I know it wipes out all your dorks, but if you're staring down Rakdos Aggro or something of that nature, you'll happily make the exchange if it can wipe the board of their threats. It'll go in the SB, but can be quite the useful tool for ya.
6) I'm on the fence about Read the Bones vs. Underworld Connections and would suggest playtesting with both. The ability to draw more than two cards with UC is great, but digging as many as four cards down with Read strikes me as optimal given your plan.
7) I would suggest upping your count of Whip of Erebos. You're currently sitting at 10 shocks, 3 Read the Bones, 3 Thoughtseize, and Erebos - that's enough damage to almost kill yourself twice (pending only one activation of Erebos). Personally, I would slide in at least one more whip as the lifelink really just makes the deck that much harder to deal with and, having run a home brew version of mono-black, you NEVER regret having one on the field. Even being able to recur threats for a single hit can be huge given the size of most of your creatures. Nevermind the life swing attached.
8) I can see where I would run some mix of Devour Flesh/Doom Blade/Ultimate Price over the full slate of 4x Putrefy. I know it seems insignificant, but why pay one more for the same effect 90% of the time?
9) Golgari Charm NEEDS to be in your SB in some number - the enchantment hate alone makes it worthwhile, but the ability to totally clear out RDW's wealth of x/1 creatures en mass is huge.
10) For whatever reason, and perhaps I'm off here, but I could see where Xenagos will have more of an effect for you than Chandra, Pyromaster. I get that she provides card advantage, but you already can dig with Read, so the mana ramping option could be more useful in a lot of circumstances - fire off that Rakdos's Return with a bit more oomph, etc.
Lastly, I'm a fan of Clan Defiance for it's multi-targeting options, Magma Jet for that added bit of scry to help with your draws, Illness in the Ranks in the SB for dealing with Elspeth/Assemble, and a second Vraska if you can swing it given that she will almost always result in a 2-for-1 or better for you.
Anywhom, I know my responses can be long-winded so hopefully you've gleaned something from all of this mess.
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