Hey Dorotheus, jchudz, Guftders and ToolmasterOfBrainerd. So, tested a Mardu rebuild as suggested.

Much more consistent flowing deck. Played against an Emeria Titan deck and went down narrowly 2:1

Hits were Zurgo for his haste and indestructability, and Dark Dwellers for the sheer value off heartless summoning, especially if they brought back either Kolaghan's Command Lightning Helix or Crackling Doom. We got supreme verdicted multiple times, and were able to bounce back quickly

Sin prodder, was a useful draw/land filter with the caveat that they're very good when you're ahead but not when you're trying to stabilise.

Eidolon was a useful 2 drop, but maybe not the right build?

Tasigur feels a bit slow. Tempted by Gurmag Angler for a bit more weight. Especially as Tasigur doesn't have haste so can be a bit slow.

I'm thinking about trying Dust Stalker and possibly Inferno Titan or Wurmcoil Engine. Dust stalker would effectively be a BR dash off heartless. It definitely feels like if we get heartless t2 we want to play a dark dwellers, flash something back and then sweep in. Malfegor might be interesting as a board wipe in combo with sin prodder.

What do people think of Mindwrack Demon off heartless?

IOK doesn't feel quite right, but I can't think of a better t1 play. Maybe Vexing Devil??...

levels of removal feel about right

Couple of other cards to think about:

Ajani Vengeant, Wear / Tear, Stony Silence, Crumble to Dust

After playtesting the deck again, (this time against my own Grixis Control deck) the deck definitely has improved. Zurgo was definitely an all star in the deck! 3 mana for a 6/1 indestructible is amazing! However, I feel like the Eidolon didn't do as much as it could...

Maybe try out Athreos, God of Passage instead? It would essentially force them to take 3 damage for taking your guys out of the picture, which I inevitably did.

Obviously, against the linear, uninteractive decks, it is an easy board out pick, as it does nothing :P

Hey Guftders thanks for testing it. Yeah, eidolon doesn't quite seem the right fit.

Will give athreos a spin as well! The other one I was thinking of was Keldon Marauders even with heartless in play you get 2 damage and a chump blocker

Spitebellows seems like it would be a pretty awesome sideboard option...

Just thought of some other awesome options. Dead Drop, Tombstalker, Gurmag Angler, Murderous Cut, Necropolis Fiend, Shambling Attendants, Sibsig Muckdraggers, and Sultai Scavenger could all be possible main deck options. Faultgrinder and Ingot Chewer for the sideboard. Some of these may be decent options for what you're going for of course you would have to balance the delve aspect. I think you ahould remove Tasigur, the Golden Fang because you can't activate his ability 9based on your current mana base) and you can't play both since he is a Legend.

Hey Squirrel_of_War some good shouts there. I'm trying Gurmag Angler over Tasigur in the next bit of testing. Also swapped in Keldon Marauders for Eidolon.

Considering dropping the IOKs for more removal. Or a couple of athreos. Also may be worth dropping a sin prodder. Maybe 1 x Wear / Tear mainboard. Although that leaves us struggling for a t1 play.

Vampire Nighthawk is great in almost any deck that runs black lots of value. Also Stony Silence is best artifact hate in modern.

Hey Draw_Wurm, your deck seems really fun to play, but I'm not sure of certain cards. You saied in your description that you play Heartless Summoning to play faster your big creatures, ans that is ok, I understand why you put it, but his -1/-1 ability don't play well with Sin Prodder and Keldon Marauders which is a creature for ultra-aggro decks, so reduce his body is not really good. So, I think that I would take off the marauders to get something bigger Also, you saied that you play delve to reduce the downside of Sin Prodder, but why not to play Murderous Cut ? Other than that, the deck seem really sweet. Great job !

I was thinking Corpse Churn would be a good card to get back a creature at instant speed and dump a few cards to use for delve.

Squirrel_of_War yeah, good idea, but I really think that the deck need more delve cards (more Gurmag Angler or Murderous Cut) to put the Corpse Churn at his full potential.

Hey Deathstokemarkov12. Stony Silence is a good shout. Always liked Vampire Nighthawk. Not sure it's quite the right fit but will playtest. The deathtouch/lifelink may make it work even if the power is reduced by heartless.

Hey marcobou. Thanks for the feedback. The thinking on Keldon Marauders is that it's 1R and always pings for 2dmg even if its power is reduced by heartless, meaning we also get to swing and chump block with a 2/2. Via heartless it costs a single red mana and theoretically should combo with Atheros, as well as giving us delve fodder, and a 2cmc draw in the late game to Sin Prodder. Anyway needs testing.

I've toyed with running one Murderous Cut, or generally some higher cost removal, but the current package feels ok.

Squirrel_of_War I like Corpse Churn but think the lack of selection could hurt us. We're also main boarding a lot of instant removal rather than Shriekmaw. I looked at Faithless Looting as well, but actually with Sin Prodder out a lot of lands end up going to the graveyard, so delving hasn't been a big problem yet (especially as heartless is often in play too).

Cool suggestions though. Much appreciated!!

Other cards to discuss as possible Keldon Marauders replacements:

Tuktuk the Explorer. 3cmc for Sin Prodder. Dies off heartless for 1 red mana to be replaced by a 4/4 and triggers Athreos at the same time.

Maybe Perilous Myr off Athreos?

The other one that might be fun is Zealous Conscripts

I 2nd the tuktuk!

I don't see the point with Keldon Marauders. If Sin Prodder sees it, then they just take two over giving you the card, which I don't think is enough to garner playing it.

Hey Bababad, Mortem. Possibly right. Thrown Tuktuk into this build and tweaked Sideboard. Need to test and see how it goes.

wow, i love the idea for using Heartless Summoning in a build like this. I may tech up my own version. maybe running Grave Pact with replacement creatures like Blade Splicer and Tuktuk the Explorer. oh yea, how about Blade Splicer for the deck! =D

+1 awesome idea!

Hey lithium142. I think the problem with Blade Splicer in this build is the relative value. With heartless out it's basically 1 mana for a 2/2 golem although you'd still get the Athreos synergy. Be interested to see what you brew up!

Anybody got any thoughts on the Dust Stalker slot? Wondering if either Hero of Oxid Ridge, Hellrider or maybe Restoration Angel for the dark dwellers synergy.

Currently got dust stalker as it's devoid, and I think we maybe vulnerable to Auriok Champion, Phyrexian Obliterator and the like because of BWR. That and we need something to strike quickly when there's an opening to support Zurgo

Meant Phyrexian Crusader...

Mindwrack Demon Was the other potentially synergistic card I was thinking of.

I personally enjoy Dust Stalker in this brew. Without HS its surprise damage with evasion. With HS, bonkers.

and unexpected for sure.

Master of Cruelties could be considered, since he doesn't need power to activate his ability? 3 drop insta win potential with HS out.

and personally...if I saw sin prodder reveal a MoC...id take the 5 most of the time

Hero of Oxid Ridge may be a good add because Sword of the Meek is unbanned.

Master of Cruelties isn't that great without power because they can just block the 0 all day.