The abyssan/undead faction can be divided into 3 flavours, by the general. Cassyva could be flavoured as a swamp/poison fog type of creature, with creep tiles being the main focus here. Lilithe is obviosuly a good fit for a Necromancer as she can raise skeletons (i.e. wraithlings) and her driving force is death (deathwatch, sacrifices etc). Maehv is more of a cthulu/chaos/demonic entity as she transforms one type of being into horror and fear beings (her bbs and abomination spell). Alternatively or additionally fungii might be considered as they thrive on decay.

(search for "*" to find groups)

*Generals:

Cassyva Soulreaper
Lilithe Blightchaser
Maehv Skinsolder

*Minions:

Abyssal Crawler					Living Swamp / Corruptor (corrupts ground into poisonous swamp) / Poison Cloud
Abyssal Juggernaut				Mucksoul / Swamp Entity / Mudglob
Abyssal Tormentor				Resonator (resonance cascade effect)
Arcane Devourer					Regurgitator (spits out another thing gor 1 mana)
Black Solus						Skeletal Construct (a pillar/moster made out of bones, every time you summon a skeleton it grows in power)
Blood Baronette					Calcinic Multiplier (calcium is good for the bones ^^)
Blood Siren						Shrieking Wight (a horrible shriek that strikes fear into enemies)
Bloodtide Priestess				Crypt Keeper (skeletons are stored in crpyts)
Bonecrusher						Swarmling (the swarm becoes stronger the more time oyu summon it)
Bound Tormentor					Cursed Mirror / Soul Stealer (self explanatory)
Cacophynos						Bloated Horror (a monster bloated from corruption, ready to explode upon death)
Carrion Collector				Soul Channeler / Endless Soul / Soul Collector (reaps souls for the master making things cheaper)
Chittering Tiller				Swamp Syren (lures enemies forward with her song)
Darkspine Elemental				Depth Dweller (deepends the swamp, making it hit harder)
Death Knell						Lich (it's a mage/arcanyst, it's undead)
Deepfire Devourer				Soul Eater (eats the souls of sourrounding minions, becoming stornger)
Desolator						Ethereal Bat / Spectral Vampire (since it transfers hp, vampirism is a good fit)
Furiosa							Bone Mother (takes care of her brood)
Gate to the Undervault			Rift Gate (because rift is in the lore)
Gibbet							Zombie (turns victims into zombies)
Gloomchaser						Skeletal Knight (comes with a free skeletal squire)
Gor								Tormented Soul / Mindless Apparition (due to a tormented mind or just being mindless it is a good fit for a non controlable battle pet)
Grandmaster Variax				Lich King (royaly screws the opponents)
Hexclaw							Cursed Sacrifice / Curse Currator (gives the enemy a curce, hitting them each turn)
Horror Burster					Spore Carrier (carries a spore that releases upon death)
Klaxon							Bubbling Cauldron / Mud Spire (releases a torrent of swamp everywhere upon death)
Moonrider						Rift Mage / Rift Seeker (makes small rifts through which reinforcements arrive)
Nekomata						Soul Linker / Death Wailer (calls forth friends upon death with it's dying breath/wail)
Night Fiend						Igniter (ignites vapours that arise from the swamplands)
Nightmare Operant				Mechanical Skeleton (because it calls a mechazor)
Nightshroud						Vampiric Mage (steals health and is an arcanyst)
Nightsorrow Assassin			Soul Searcher (searches through your soul to judge if you are a worthy opponent)
Nocturne						Vapour Shaper / Living Swamp (makes monsters out of swamp gasses)
Ooz								Oozing Fungus (oozes mud and decay)
Phantasm						Soul Watcher (watches what the enemy is doing, giving souls to its master)
Reaper of the Nine Moons		Trapped Soul (it's a soul trapped in the minion body, upon death it is given release)
Shadow Sister Kelaino			Succubus (not really a vampire but close enough for goverment work)
Shadow Watcher					Gatherer of the Dead / Soul Carrier / The Boatman (gathers dead souls and the boatman is a reference to the greek myth of the Styx Boatman)
Shadowdancer					Vampiric Gazer (it watches for death and it is a vampire)
Skullprophet					Diseased Carrion (makes the opponent general sick)
Spectral Revenant				Bone Dragon (wow such original much effort)
Stygian Observer				Well of Souls / Soulstorm (a buncha souls everywhere)
Underlord Xor'Xuul				Riftmaster (uses the rift to multiply the dead)
Void Talon						Waiting Gargoyle (it's a statue at first, then it becomes a monster)
Vorpal Reaver					Bone Giant (explodes into normal skeletons)
Xerroloth						Rift Conjurer (conjures reinforcements from the rift)

*Spells:

Abhorrent Unbirth
Aphotic Drain
Betrayal
Blood Echoes
Breath Of The Unborn
Choking Tendrils
Consuming Rebirth
Corporeal Cadence
Daemonic Lure
Dark Seed
Dark Transformation
Darkfire Sacrifice
Deathfire Crescendo
Deathmark
Demonic Conversion
Doom
Echoing Shriek
Grasp of Agony
Horrific Visage
Infest
Inkhorn Gaze
Inkling Surge
Lurking Fear
Munch
Necrotic Sphere
Nether Summoning
Nethermeld
Obliterate
Painful Pluck
Punish
Rite Of The Undervault
Ritual Banishing
Shadow Nova
Shadow Reflection
Shadowstalk
Soulshatter Pact
Sphere of Darkness
Unfathomable Rite
Unleash the Evil
Vellumscry
Void Pulse
Void Steal
Wraithling Fury
Wraithling Swarm
Yielding Depths

*Artifacts:

Furor Chakram
Ghost Azalea
Horn of the Forsaken
Mindlathe
Soul Grimwar
Spectral Blade
The Releaser
Wraithcrown

*Tokens:

Abomination
Fiend
Horror
Husk
Void Relic
Watchful Sentinel
Wraithling