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Skeleton Minion is very strong for an at-will - having an unlimited chain of minions is a bit much.
Leech Life has nonvariable, fixed damage - this is bad as it can't be enhanced. That means the power's damage will not scale correctly as you level (it isn't as critical for encounters and dailys as they're not supposed to scale). As a starting power it's extremely underpowered, and it will just get worse.
Poison Burst is also a bit strong. Ongoing damage is generally considered too good for an at-will, although dropping it to 2 helps there. Ongoing plus area burst is definately too much.
Withering Strike is a melee attack, it probably shouldn't count as a ranged basic attack. Since it also gives temporary hit points, it's good enough without counting as a melee basic attack - just leave the special off.

The encounter and dailys have recuring several issues that should be worked on. I would suggest you read p.59 on Conjurations and Zones (none of your Zone powes have the Zone keyword) and then take a look through the Cleric section to see examples of level appropriate things you can do with a conjuration spell.

Just right off the bat you combine too much into the one class. Just the features aren't even balanced against a wizard. Armor prof up to chain, and more hp, more surges, a free melee based implement makes for a large advantage.

Make a magic item like a pact blade for scythes to keep it more consistent.

Almost all of the attacks are Int based, with one Cha, and one Str. There should be either two stats for the at-wills with a more even split, or keep it just Int. Could do Int and Str with Cha buffing some moves like the features etc.

Aura of Dread: Npc's don't make death saving throws, and and a -1 to all def in a 81 square area (I think) is huge. I'd just take the whole thing out it's a bit more powerful than cantrips and a spellbook.

Corrupting Essence is interesting, but I'd almost rather see it as a status effect for one round like weakened. it should probably be more like the wizard implement feature, and be 1/enc with no negative, then you could do damage with a weaken.

Death Knell: Seems like it should actually be some kind of at-will power, more later.

Having a skeleton as a class feature that you can spend actions to command seems like it should be something they have. Summon it during a short rest, it takes a healing surge and has that many hp, give it a range limit of 20 so it can move around a bit. It can make OA's and flank with people, but uses your actions to attack, basic attack with your weapon type. I think allowing it to move when the necro does might be ok. Have encounter and dailys that can do effects around skeleton as an add on. Necro can use Str or Int for Skelly, choose at char creation.

At-Wills:
Withering Strike: Make it necrotic damage, with keyword. Replace temp hp with a debuff to fit the name better maybe a -2 to hit for a turn. Make the skeleton able to use it in place of it's basic attack.

Poisonous Burst: Maybe make the ongoing damage equal to 1/2 Int for scaling. Remove necrotic keyword.

Death Knell: Str vs. Fort, Weapon keyword, Melee attack. Curses target with visions of it's impending doom. 1[w] necrotic damage (no Str), target takes a -2 to a defense of your choice and counts as bloodied to you an your allies till end of your next turn.

Aura of Dread: Maybe has a place here. Make it a close burst 1 (all creatures) fear power, targeted around Necro or Pet they are both immune to the effect. Int vs. Will: 1d6 + Int psychic damage, and push Cha squares.