Fixed scaling to bring it in line with other at-wills. Damage dice should not shrink at level 21! :3
Oops, must've been a copy/paste error.
This would be necromancer C.
Yeah, it's C. Sorry for the confusion.
Grave Pulse could use a little work. As it stands, its underpowered and a little boring. Consider bumping it up to an area burst 1 attack with a possible sliding effect. That would balance it out with the weak damage output.
Ghastly Bite could be upped to 1d8 as well. Makes it a bit more appealing.
Tomb Guardian Strike is a cool power, but it could also benefit from a slight damage boost. Make it a 1d6 and make it a comparable power. Also, we're missing build clauses for encounters/dailies it seems…
Swarming Horde, the verbiage should be "and you slide the target 1 square."
Swarming Horde should specifically say "no more than one attack per target" in the target, otherwise you can abuse the ability to have 8 minions around a single target to manage obscene damage. Similarly, because you are making so many attacks, you may want to consider removing the extra d6 at epic tier and instead replacing it with a flat 2 or 3 additional damage. It would also make more sense for the power to instead be written in this way instead
Close burst 10
Targets: All of your minions within burst
Effect: The target makes the following attack, no more than one attack against a single target
Minion 1
Attack: Intelligence vs. Reflex
Hit: Intelligence modifier damage and you slide the target 1 square
Bone Claw provides some incredibly powerful area denial and damage capability because you can have so very many minions out at once. I would instead have the power slow the target hit by the power instead and have the claw growth simply be used to explain the extended range.
Deathly Shriek is a bit too strong considering it's fundamentally identical to the Wizard's Thunderwave. Deathly Shriek is discriminate, does the same damage at a minimum, has the potential for more, and targets a normally lower defense (Will is generally the lowest defense). I would make the attack still be able to hit allies (because you should still be able to scare the hell out of your friends) and remove the additional damage capability (turning it into a psychic/fear version of thunderwave) or remove the 1d6 damage on a hit (turning it into Int mod with a potential Cha mod).
Spread sickness has too much capacity to be broken because you can spread effects endlessly (because they last until the end of your next turn). If you had a creature that was stunned, you could stun it and one of its friends ad nauseum. I would instead suggest that you have the power deal additional damage to a target's allies if it is taking ongoing damage or give the target a -2 penalty to saving throws until the end of its next turn (while changing the name from spread sickness to something like "Intensify Sickness").
Also, you probably want to have one or two powers available that can be used as a ranged basic attack (or simply provide a power as a class feature that can be used as a ranged basic attack: Necromancer's Blast, Ranged 10, Int v. Ref, 1d6 + Int mod necrotic damage).