You have too many cards that have narrow focus and require Horobi, Death's Wail . You want to minimize the amount of these cards. Things like Belbe's Armor , Staff of Nin and Icy Manipulator are fine because they are still useful without Horobi, Death's Wail . However, things like Squee's Toy , Infused Arrows and Scale of Chiss-Goria are too reliant on Horobi, Death's Wail to have value.

~Side note, remove Infused Arrows immediately! Removing a charge counter is part of the cost of the effect and you will only be able to cast it with one charge counter (you can only produce black mana with this deck).

Liquimetal Coating , is good if your meta looks at artifacts a lot or you have other interactions with artifacts. If you are JUST using it for the combo with Horobi, Death's Wail then take it out. The reason why I still include it is to get rid of land when paired with Xenic Poltergeist , or to kill other noncreature permanents with Xenic Poltergeist and another target spell with Horobi, Death's Wail out, or other creature removal.

Don't completely focus your deck on Horobi, Death's Wail . You'll find he will be killed A LOT if you focus him, or even upset your opponents' plans too much with him (he's very easy to deal with). You want to use him to throw a wrench in the most OP deck's plans so that your other opponents just play around him and make that OP player focus their resources on him. 9 times out of 10 you just want to sit there, build up and let your opponents take out themselves first. If you don't show that you're much threat (which you really can be with Horobi, Death's Wail ) then they'll leave you for later.

My meta isn't a fan of infinite combos, so I don't use any. However, if your meta uses them, you need to focus the players that have them and prevent them, and you should probably include some of your own. If you don't know any mono-black combos :

Mikaeus, the Unhallowed + Puppeteer Clique + Altar of Dementia (the first 2 do work on their own too)

Mikaeus, the Unhallowed + Triskelion

Gravecrawler + Vengeful Dead + Phyrexian Altar

Bloodchief Ascension + Mindcrank

Exquisite Blood + Sanguine Bond

Blood Tribute + Sanguine Bond (single target, instant death)

Wound Reflection + Blood Tribute (single target, instant death)

Grave Titan + Nim Deathmantle + Ashnod's Altar

Profane Command & Dauthi Embrace are auto-includes with Horobi. Expedition Map will be necessary when you add some lands worth tutoring...

Land Base: Cabal Coffers Dark Depths Desert Hall of the Bandit Lord Maze of Ith Nykthos, Shrine to Nyx Shizo, Death's Storehouse Temple of the False God Urborg, Tomb of Yawgmoth Volrath's Stronghold

Other than Sorin Markov, I like your deck.

Baleful Force Disciple of Bolas Erebos, God of the Dead Bloodgift Demon Moonlight Bargain & Decree of Pain are all great card draws. When I play-tested you seemed to be lacking.

Kuro, Pitlord is expensive, but with Horobi, He wins Games. If you were running some token producers, you could run this: Contamination. It is just great with Ophiomancer & Abhorrent Overlord.

My BLACK, Deck of Lords and Titles is ran with cards that are useful either way. I did not build around Horobi, too much. You should check it out. I could use some fresh suggestions and discussions, myself.

Since you lack card draw, try using Commander's Sphere instead of Darksteel Ingot. It will help you late game.

Darksteel Colossus should probably become Blightsteel Colossus. More killing power.

Kozilek, Butcher of Truth should probably be an Ulamog, the Infinite Gyre since he has indestructible, survives your commander.

Liliana Vess is only really good for her tutor. Try using Diabolic Tutor or Vampiric Tutor instead, since she rarely lasts more than a turn.

Brittle Effigy seems a bit overcosted. Unless you see a lot of indestructible effects, ones that you can't hit with your sacrifice effects, it probably isn't really worthwhile.

I personally dislike Armillary Sphere, typically Expedition Map is a better choice. It curves much nicer if you have it in your opening hand, plus it finds Cabal Coffers

Harvester of Souls might give you some needed card draw.

Sepulchral Primordial is a great way to close games, steal their best creatures then attack later.

Nim Deathmantle is one I always recommend when lots of creatures are dying. It lets you reuse creatures and get more ETB triggers. It also means that you don't have to recast your general if he dies.

Necropotence is a dirty card, but it also gives so much card advantage. I would recommend it, but use it with caution. Some people I know use it liberally and decrease their health, then die to all the aggression of other players.

I use Kozilek for repeated card draw with Eye of Ugin. So his lack of indestructible is an upside. It works surprisingly well.

I have a lot of Bane of Progress in my meta so I like Darksteel Ingot, but I suppose I could just sac it for a card.

I secretly cut Necropotence because it wasn't working the best. I played a lot of life, just to lose tempo. the having to draw the cards at end of turn slowed me down too much.

Darksteel Colossus if filler until I find a blightsteel for trade.

Lili sticks around surprisingly often. Plus it's what I had on hand.

I have both Armillary Sphere and the map in. I like the land ramp especially with Cabal Coffers

Nim Deathmantle always seemed high cost, but seeing I can loop Kozilek with eye, maybe isn't that bad.

I have a Grim Haruspex would that be a suitible swap for harvester?

Thanks for your help.

And I cut Tragic Slip for Brittle Effigy, do deal with indestructible, like ulamog and things like Iona.

But Tragic Slip deals with indestructible things too, doesn't it? And it's a lot cheaper. Plus the whole exile thing doesn't do much when Horobi is on the field, since it targets. Unless the effect sticks when Horobi fails to kill an indestructible???

Tragic Slip only deals with indestructibles of toughness less than 13 and only if a creature has died this turn. Also, if Iona or her ilk is out it is useless.

The effigy is more expensive, but it is worth it.

Yes, it would still be exiled.

Helvault has been stellar in my playtests. It provides removal and protection from exile and tuck effects.

For the Grim Haruspex vs Harvester of Souls, it really just depends on if you plan on drawing cards for your creatures or for your opponents creatures. I haven't really played this deck so I don't know how it would go in multiplayer, but I believe you would draw many more cards using Harvester of Souls. Or you could just use both, that is always an option.

What do you suggest as removals for the pair and the mantle?

Only cards I can see that should be removed or don't fit the theme are Pawn of Ulamog (draw cards with Grim Haruspex instead of getting 1 mana) and Ugin, the Spirit Dragon (he doesn't seem necessary, you have enough boardwipes already and he costs a lot)

I agree with pawn but Ugin can deal with enchantments, something that black fails at.

I normally run Nevinyrral's Disk or Oblivion Stone for enchantment/artifact hate,

Exile removal with an option to target makes Ugin worth his slot.

Although Oblivion stone targets.

Oblivion Stone is a solid addition.

I'll add it once I can pick it up.