MTG Salvation

As a note on Necromare, it does not make two tokens a shot like Mitotic Slime does, only one, so it's a dwindling power rather than a sustained one.

I don't like the contagious thing. Someone shooting an infected person from a distance is not going to contract what they are carrying. Further, depression, lethargy, and those types of mental states are generally blue in color, rather than white. To get a proper wording on that reminder text, as well, would pretty much make the whole text box a reminder.

Screeching Roost XBBB
Enchantment - Aura
Enchant land you control
~ enters the battlefield with X roost counters on it.
At the beginning of your upkeep, you may remove all roost counters from ~. If you do put X 1/1 black Bat creature tokens with flying and haste onto the battlefield under your control, where X is the number of counters removed this way.
At the beginning of each end step, exile all bats you control, and put that many roost counters on ~.
BB, Exile two bat creatures you control: put three roost counters on ~.

I think there is a problem with fight affecting multiple creatures at once. For one, its all one ability, and therefore lethal damage is not checked until after it resolves. And fight isn't divided damage, its each creature dealing its power in damage to the other. So for 7 mana, your zombie is shooting everything your opponent controls for 7 damage (chances are, wiping their field clean) AND netting you a 2/2 zombie token for every creature they have that didn't manage to survive that onslaught. And with the natural affinity zombies have for reanimating, it would be a repeatable board sweeper.

The octopus, I like. But I might reverse it so they opponent can have their creature fight, and if they don't THEN that creature is tapped. Either fight off the tentacles or get snared by them.

The Night Watch feels odd as well. Kind of an off-beat vigilante hero. Crime boss (player) shoots his sidekick, he beats up one of that boss's thugs.

Fairly costed, I'd say. Any deck can run it for 4, mono can run it for 3, and blue/green gets it for 2.

Necromare 1BBB
Creature - Zombie Horse
When ~ dies, put a 2/2 black Skeleton Horse creature token onto the battlefield under your control. This token has "B: Regenerate this permanent" and "when this creature dies, put a 1/1 black Spirit Horse token with flying onto the battlefield under your control."
You just can't beat a dead horse.
4/4

Here is my take on a new Sorin, with inspiration from his fellow vampires:
+1 Put two 1/1 black bat creature tokens with flying onto the battlefield under your control.
Garruk 2.0 gives you a 3/3 for +1, this gives you more blockers/attackers, but less power.
-4 Destroy target creature. It can't be regenerated. You gain life equal to its toughness.
Sorin goes to town on a creature, but saves you some.

Finally, if you must have a tribal touch...
-7 You gain an emblem with "creatures you control are black vampire creatures in addition to their other types and colors, and gain Flying and 'whenever this creature deals combat damage, put a +1/+1 counter on it'."
Not quite a permanent Mephidross Vampire, but can certainly turn the tide of a fight, as a final should. Especially potent with the first, building up a swarm of bats to swing with.
{5}

I might lower the cost to just RR, 1RR at the most. Unless playing a time counter-based deck, your opponent has 3 turns to kill it without repercussions. Or to get you to a point where it will kill you for them.
Even used with a Brute Strength (I think that was the red Giant Growth variant), you and your other creatures are taking that hit too, so it may still do you more harm than good to use it.

Mass Mimicry UU
Sorcery
Each player reveals cards from the top of his or her deck until a permanent card that shares a name with a permanent he or she controls is revealed, then puts that revealed card onto the battlefield under his or her control. Shuffle the remaining revealed cards into their owners' libraries.

The name and templating need work, but the gist is that everyone gets a permanent out of it (unless they are in a weird little situation where they are only running one of every permanent in their deck, or the other copies are already out of their deck). Prevented from being too abusable by using the Mitotic Manipulation control of only allowing a card with a shared name. So no flipping to a singleton Emrakul for 2 mana.

Well, without proper wording, it seems broken as hell. 7 mana exiles all creatures, gives YOU token copies of them, and when those copies vanish, YOU get the originals back as well.
Exile all non-token creatures. For each creature exiled this way, its controller puts a creature token that is a copy of that creature onto the battlefield under his or her control. Each of these tokens gains "when this permanent leaves the battlefield, put target exiled creature card that shares a name with it onto the battlefield under your control."
At least that is how I think it would be worded to get what you are shooting for.

Delectable Meepling GG
Creature - Beast
All creatures able to block ~ must do so.
Whenever ~ is blocked, target creature blocking it fights another target creature blocking it.
A delicacy to die for... or to kill for.
1/1

For reference to what a meepling is, see Irresistible Prey and Prey's Vengeance.

Honestly, I would never run it. Landwalk is such a situational ability, and spending mana and tapping one of my attackers to replace it with another situational evasion doesn't seem like it would be that useful. If I were that keyed up on my swampwalk creatures, I would just bring a few auras to MAKE them have swamps, or just use Urborg, Tomb of Yawgmoth.

Also, T is not mana, and can't be added to your mana pool.

Elm-bind Ritual 1GG
Sorcery
Target creature becomes a Forest land.

This is not an 'until end of turn' effect, nor is it 'in addition to its other types'. The goal here, thematically, is to seal the creature inside a tree. This is partially inspired by Mercy Killing and Beast Within, which both gave green targeted destruction for non-artifact, non-enchantment permanents, at the price of giving your opponent something. In this case, they get a land. Costed the same as those two because of sorcery status (instant versions usually cost 1 more.

It is basically Master of the Wild Hunt, without the token factory ability, and a changed method of wolf-ganking. I might almost change it to give all your wolves an activated fight ability, but still interesting and fair, at least.

Last Man Standing 2R
Enchantment
At the beginning of each player's end step, that player chooses a creature he or she controls, and another creature. The chosen creatures fight each other.
If a there are less than two creatures on the battlefield, sacrifice ~.

Actually, I think its better than a lot of the cards in Innistrad, short of MAYBE the names for both sides. Not over-powered, but interesting enough to build toward/around. Maybe add in a reverse flip for when the riots 'die down'?

Eternal Unrest 2BBB
Enchantment
Whenever a non-Zombie, non-Skeleton creature with power 2 or greater would die, instead exile it, and its controller puts a black X/X Zombie creature token onto the battlefield under his or her control, where X is half that creature's power, rounded down.
Whenever a Zombie creature with power 2 or greater would die, instead exile it, and its controller puts a black X/X Skeleton creature token onto the battlefield under his or her control, where X is half that creature's power, rounded down.

I hope I got the wording right. The goal being, when something dies, it stands right back up, weaker but still moving, until even its bones are completely destroyed. The 2 or greater clause is there to prevent a 1/1 creature from dying, giving you a 0/0 zombie that auto-dies, followed by a 0/0 skeleton that does the same. The exile replacement clause is for flavor and shenanigan prevention with the graveyard. You're already going to get your creature back (somewhat) twice anyway, once if it was a zombie.

In honor of Innistrad, and a famous series with a similar setting:

Vampire Killer 4
Legendary Artifact - Equipment
Equip 2
Equipped creature gets +2/+0.
If equipped creature would deal damage to a Werewolf, Zombie, or Vampire, it deals double that amount of damage instead.
"From this day on, the Belmont Clan will hunt the night." - Leon Belmont

Megaman 2UW
Legendary Artifact Creature - Construct
UW,T: ~ fights target creature an opponent controls. If that creature dies this turn, exile it instead. ~ gains all activated abilities of creature cards exiled this way.
3/4

I decided Blue/White both because he IS blue, and because the blue/white philosophy of maintaining the status quo. All his fighting was aimed at protecting the world from evil and chaos. The goal in the ability is something similar to Myr Welder, but gaining abilities via fighting rather than dumpster diving.

I think the Swarm of Boos needs '~ can't attack if an opponent controls an untapped creature' or something. For the whole, not moving when looked at bit.

Honestly, I think the ability is backwards for black. Losing two creatures for one is a bad investment, and black is generally about getting the most out of whatever it can. Saccing a human to make a spirit token and zombie token would be more in-line with the color.

Prophesied Demise UB
Sorcery
Choose a creature. Until the beginning of its controller's end step, if that creature dies, Scry X, where X is that creature's converted mana cost, then Draw a card.

Needs to cost more. Wheel of Sun and Moon puts it straight to the bottom of the library, while this puts it randomly higher, possibly even top-deck or close to it.

Mana Beast xUU
Creature - Elemental
~ enters the battlefield with X +1/+1 counters on it, and Y charge counters, where Y is the amount of U spent to cast it.
Protection from converted mana cost Y or less, where Y is the number of charge counters on it.
Pure magic, with pure violent intent.
0/0

For the sake of example, spending UUU to cast it gives you a 1/1 with protection from cost 3 or less, while 8UU gives an 8/8 with protection from 2 or less.

The thing is, that final in the first post doesn't require the opponent's creatures to fight YOURS, just another creature. So if your opponent controlled a 10/10 and 10 1/1s, you could have all the 1/1s fight the 10/10, killing all of them. So that final has potential to be a Plague Wind.