There are a couple things I would like feedback on:
Mageripper Swarm
Level 6 Controller
Medium Aberrant Beast
XP 250
Initiative +6
Senses Perception +11
Swarm of Fangs aura 1;
The Mageripper swarm can make a bite attack as a free action against any creature in the aura.
HP 72; Bloodied 36
AC 20; Fortitude 18, Reflex 18, Will 18
Immune Gaze, Illusions; Resist Half damage from melee and ranged attacks;
Vulnerable 5 close and area attacks
Speed 6, climb 4
Bite (Standard; at-will)
+11 Vs. AC, 1d10+4 damage
Anti-Magic Shield (Immediate Reaction (when hit with a magic attack); 1 per round)
The attack is changed to a miss
Magic Leech
When missed with a magic attack, the target loses the use of that attack until the end of the encounter.
Alignment Unaligned
Languages None
Skills Athletics +11, Nature +11, Stealth +11
Str 16 (+6) Dex 16 (+6) Wis 16 (+6) Con 16 (+6) Int 16 (+6) Cha 16 (+6)
Equipment None
This is an interesting creature but my gut reaction is to say that it should be paragon level with those sorts of abilities.
Swarm of Fangs should probably be the standard "all enemies starting their turn inside the aura take a Bite attack", i.e. ite aura attack happens on the players turn rather that the swarms.
Anti Magic Shell doesn't need the "1 per round" as a creature can only have 1 immediate reaction in a round anyway.
This ability in effect is very similar to the Mindflayer Masterminds "Cradle of the Elder Brain" power, a power that is very good, and thats a level 18 elite.
Also the term Magic Attack is a little vague, you are probably refering to powers with the Arcane keyword, and maybe the Divine keyword as well but there is the possibility of stretching the term magic attack to incorporate magic weapons (which I would say isn't intended) so it needs a little clarification.
All in all I think this power is too strong for a level 6 standard creature, it probably wants to be changed to an encounter power, or if the creature is an elite changed to a recharge


Magic Leech is a great power for a paragon or epic tier creature.
I see what you are trying to do with the creature and its a good idea but I think for heroic tier it will need to change its focus. As it is an anti arcane creature it could maybe do more damage to arcane powered creatures or maybe it gets a bonus to its defences from arcane attacks or gains bonus damage on an attack when it has been hit by an arcane attack, or when hit by an arcane attack has a free attack to deal damage equal to half the total back at the caster?
NOTE: I am not sure if all of these fit a controller.
Magebane
Whenever Mageripper Swarm hits a creature with an arcane power source
it deals an additional 1d6 damage
Arcane Resilience
Mageripper Swarm gains a +2 bonus to all defences against attacks with
the arcane keyword
Arcane Battery
Whenever Mageripper Swarm is damaged by an attack with the arcane
keyword its next successful attack deals an additional 5 damage and the
target is dazed (save ends)
Reflective Shell (immediate reaction, recharge


attack with the arcane keyword
When hit by an attack with the arcane keyword Mageripper swarm reflects
part of the attack back to the caster; attack +10 vs. Reflex; on hit target
takes damage equal to half of the damage dealt and any status effects
generated from the attack (note: Mageripper Swarm still takes full effect
of the attack)
anyway these are just ideas.
Necrosis Carnex
Level 3 Skirmisher (Leader)
Medium Natural Animate (undead)
XP 150
Initiative +5
Senses Perception +8
Malign Aura (Necrotic, Fear) aura 3; All living creatures within 3 squares of the Necrosis Carnex take a -1 to all defenses.
HP 46; Bloodied 23
AC 17; Fortitude 15, Reflex 15, Will 15
Immune Necrotic, Poison, Disease, Sleep;
Vulnerable Radiant 5
Speed 8
Necrotic Touch (Standard; at-will)
+6 Vs. Will, 1d10+3 necrotic damage.
Necrotic empowerment (Standard; recharge
)
Burst 5, all allies in the burst can make a basic attack as a free action.
Unholy Explosion (Immediate Interrupt, when reduced to 0 hitpoints) ♦ Necrotic
Burst 3. all living creatures in the area are hit with the following attack. +6 Vs. Reflex 3d6+3 necrotic damage and all undead allies in the burst regain 15 hit points
Alignment Evil
Languages Understands Creator
Skills Acrobatics +8, Athletics +8
Str 14 (+3)Dex 14 (+3)Wis 14 (+3)Con 14 (+3)Int 14 (+3)Cha 14 (+3)Equipment None
I quite like this creature, it is potentially abusable (but thats not really our problem).
Necrotic Empowerment is really rather strong for a level 3 creature, I would say it should probably read all undead allies in burst. At this level on a standard creature I think it should probably be an encounter power or changed so that it only effects up to 2 allies. Otherwise level 3 PCs are going to get pummelled.
I like unholy explosion but 3d6+3 damage could be pushing it at level 3.
I think this creature should also have another attack power to keep it interesting.
Void Shard
Level 3 Minion
Tiny Shadow Animate
XP 37
Initiative +3
Senses Perception +3
Withering aura (Necrotic)
aura 2; All creatures in the aura take 1 necrotic damage. Multiple withering auras stack (void shards are immune to this ability)
HP 1; a missed attack never damages a minion.
AC 17; Fortitude 16, Reflex 16, Will 15
Speed Fly 6 (hover)
Void touch (Standard; at-will)
+6 Vs. Will, 1 necrotic damage
Of another dimension (Immediate Reaction (when hit by an attack); at-will)
There is a 50% chance that the attack is changed to a miss.
Portal (Immediate Reaction (when missed by an attack); at-will)
Another void shard appears in a square adjacent to the activating void shard
Alignment Unaligned
Languages None
Str 16 (+4)Dex 16 (+4)Wis 14 (+3)Con 16 (+4)Int 14 (+3)Cha 14 (+3)Equipment None
Some of these powers seem familiar

I think this is proably balanced due to the very low damage, and the cumulatibe aura damage. In fact you might want to drop these down to level 1 minions for use in encounters from levels 2 to 4.
anyway all this is just my opinion, hope it helps.