I'm not really sure what you're going for. You've got Horobi, and the classic keep-you-down cards like Dauthi Embrace, Caustic Crawler, etc., but then you have cards like Lifeline, or really pricey creatures like Pestilence Demon or Lim Dul that will die to your opponent's targeted stuff and leave you with a tempo loss. Perhaps my analysis is off. You've got Contamination in there, but no Reassembling Skeleton to make it stick without using the horribly symmetric Lifeline (which basically undermines the whole creature-kill project of Horobi, and makes Contamination less of a 'lock' and more of a static board state where people might kill you); pestilence and pestilence demon who are basically redundant in Horobi (because stuff just dies when you want it to).
Aphotic Wisps will not do the work you want it to; it never actually resolves (because all targets are gone, having changed zones) to draws you a card. If you're interested in cantrip kill, Death Pulse and Dirge of the Dead, both from Onslaught are what you're looking for. For those cards, the kill is an after-effect of the cycling ability and its own independent object on the stack, so you will still draw a card for cycling these spells. The triggered ability will fail, but, it wont matter, because it's solely some targeted effect, and has no other effects riding on it (like the card draw does on Aphotic Wisps).
Shriveling Rot and Falkenrath Noble are pretty much wasted slots unless you expect your opponents to have a lot of permanents hitting the dirt. Massacre wurm will kill people who have a multitude of tokens all on his own; help from Shriveling Rot or Falkenrath Noble is basically unnecessary. I find this sort of nickle-and-diming doesn't really work in EDH; people die to the 5/5+ creatures and the huge combos that don't depend on a particular board state from opponents, rather than a couple of damage here and there (though, Ascension warrants inclusion because it nickle-and-dimes people for so everything, and the number is significant).
Sword of the Paruns is probably a bum slot; Horobi kills people for equipping stuff. I'm not sure who you plan on tapping or untapping a ton. I suppose it could be part of a combo, but I can't see it. Blowfly infestation may not work as well as you want; I'm not sure how you'll get -1/-1 counters on creatures without targeting them.
For combo, you're missing a ton of tutors. You could be playing Demonic Tutor, Vampiric, and especially the Dimir transmute cards from Ravnica. There are transmutes for almost every CMC; Shred Memory, Dimir Machinations, Dimir House Guard, Brainspoil (maybe not this one) and Netherborn Phalanx. You're also missing some graveyard hate; you could be running Bojuka Bog and Nihil Spellbomb very painlessly. Urborg, Tomb of Yawgmoth is highly useful with Coffers.
For other creature kill that's painlessly run, I used Urborg (from legends), Desert (Time Spiral Shifted, Arabian Nights), Shizo, Death's Storehouse (Champs of Kamigawa) and Tower of the Magistrate (Mercadian Masques).
For value in this kind of game, planeswalkers really break the bank; a toxic environment for creatures leave planeswalkers relatively uncontested.
I hope this helps. It's not my intention to be harsh, but I have trouble seeing the combos lurking in your list.
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