I really like the themes and interesting effects a lot of these cards have but I see a few problems.
First the land is quite overpowered, and I assume the ability is meant to search for a swamp or mountain? But just tapping for 2 mana on turn one and being a dual land that enters untapped under no conditions is ridiculously good, without having free mana fixing.
The enchantment that gives your guys haste seems very underpowered considering, Fervor is a 3 cost enchantment that gives all of your creatures haste not just one specific creature type. Maybe give it something to buff your Enraged Phantoms as well.
Kankrellaij might be able to be a 3/3.
Material Breakdown is good, I just think it is more in the Rare range not Mythic.
The Defender seems to go against the overall playstyle considering it can't attack. But other than that it's a very good defender.
The Blackgate Leviathan is my favorite. It is absolutely brutal. But I think the wording should be. "Whenever ~ destroys a creature as a result of Combat Damage, you may return an Enraged Phantom creature card from your Graveyard to your hand." (I could be wrong)
The artifact is a really cool way to cycle your creatures and lands, and basically ensures you will always have creatures to play. Might be Mythic.
The Planeswalker seems pretty balanced, IMO.
The last card is interesting... placing the minus 1 counters on it seems relatively unnecessary, as I don't see it surviving much combat with 2 toughness anyway. Also I believe Vigilance is more of a White ability, probably not for Black/Red. Could just give it Haste instead, and make it a 6/2. 5 cost 6/2 Haste seems pretty good for a common, IMO.
One big thing I also noticed is that you say "Bury" on most of the cards instead of "Destroy". I don't know if you did it on purpose or if you just like the older wording of Magic Cards, or if i'm totally in the dark and Bury actually means something different than Destroy. But having only played Magic for about a year and a half I feel like the wording could be cleaned up a little bit. (It's no big deal though)
Overall I really like your cards here.