It would probably be better if you started a discussion thread for each of these cards. Also, please be mindful that we request a "current wording" be included with each card, to keep track of changes from the initial proposal. I will make my commentary on all of them here.
Crazed Reaper - I really like this card. It's hard to design metamagic for red, but I think that this card does it well. If you want to up the activation cost to "remove three counters" then you could easily allow the 2 damage to be split between targets.
Dark Brute - I think that this card is a too powerful. A 4/4 creature for four mana is not exactly bad, but then you have added on a no-activation-cost Engineered Explosives to it. Black has plenty of ways of sacrificing a creature that it would not be necessary to have this die in combat. It is simply going to sit on the table waiting for an opportunity for you to sacrifice it. In that regard, it reminds me a lot of Angel of Despair, which costs

more than this does. Although this is weaker than Angel of Despair in several ways (smaller, doesn't fly, requires another card to be really effective), the fact that this can also take out numerous permanents at once helps it regain a lot of ground. I wouldn't consider costing this any less than 5 mana -- maybe


{B}{G}.
Obsessive Wolf - I don't like this card at all. It is an enchant land hiding in a creature's body. Keeping track of which land this affects is awkward, because you're not going to leave the creature underneath the land card. Your templating also makes it ambiguous as to what this actually does. Do you choose a single land in play or do you name a nonbasic land and then all nonbasic lands with that name cause damage when they tap? If you wanted to make this an enchantment, I could get behind it a lot more. Psychic Venom never really belonged in blue to begin with.
Volcanic Curse

Aura
Enchant nonbasic land.
Whenever enchanted land becomes tapped, Volcanic Curse deals 2 damage to that land's controller.