Since you only have black in the deck, you don't need the Savage Lands, plus since your mana curve is so low you don't even need to replace them with other lands. Personally, I'd throw in one Grave Titan just to have a creature engine. Also, in conjunction with Souless One it would be really good. Those ar two cards any Zombie deck must have. Skirk Ridge Exhumer is also a good creature engine.
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Maybe you could add Azusa, Lost but Seeking, Budoka Gardener, or Walking Atlas.
This deck is interesting, because usually a deck without 1 and 2 drops would die, but with cascade, I don't know if that is a problem.
Canopy Cover is a great green anti creature destruction for 1 colorless and a green. it gives an enchanted creature shroud except only your opponents can't target it. Aspect of Mongoose is almost as good. it cost the same and gives the enchanted creature shroud, but when Aspect of mongoose is put into the graveyard from the battlefield it is returned to your hand. And a card you should definetly put in the deck is Vexing Shusher. It can't be counter and costs 2 red or green. It has an activated ability that cost a red or a green that says "Target spell can't be countered by spells or abilities." That will hopefully fix the counter spell vulnerability your deck has.
I played out a game on paper and with the sample hand and what I found is that it is really easy to get mana with this deck, but there is not much to do with it. Maybe throw in something that a player can actually do
This deck looks really good. The only thing that might present a problem is A: Counter spells which annoy everybody and B: If you run into something too big for the deck to handle since the only answer you have for anything big is Scute mob and maybe Dragonmaster Outcast.