I've been thinking about going through the Army Books one by one, and updating the Lores to something akin to the 8th Edition Lores.
I tried something similar
a few weeks ago; I have not play-tested it yet.
Warriors of Chaos: Nurgle[indent]Vile Affliction (Lore Attribute)
Sorcerors using this Lore have Poisoned attacks. In addition, all direct damage (Except Magnificent Buboes and Cloying Quagmire) and magic missiles cast by the Sorceror have the Killing Blow special rule, as not even the toughest foe can withstand a heavy dose of an horrendous disease.
More aggressive than my idea; however there are not that many direct damage spells so it does not horrify me.
Cloud of Flies (Signature Spell) (10+)
Augment. The bearer summons a cloud of flies to cloak him those surrounding them in a cloud of fat poisonous biting bugs. Once cast, this spell is in effect for the rest of the game. Although it acts like a Remains in Play, it's the spell which summons the flies, rather than keeping the flies present.
The Sorceror and any unit he joins counts as being behind a soft cover and opponents suffer an additional -1 to hit in close combat. This stacks with the Mark of Nurgle.
Making it remain without being Remains in play seems a little powerful; as the spell summons flies maybe it should last a number of turns instead of indefinately.
1. Magnificent Buboes (5+)
Direct Damage. Pick a single model within 24" within Line of Sight of the Caster. It takes a wound with no armour saves allowed.
The sorceror can choose to extend the range of this spell to 48". If he does so, the casting value of Magnificent Buboes is increased to 8+.
2. Fleshy Abundance (7+)
Augment. Choose a friendly unit, even if in combat. If that unit is within 18" of the caster, it has Regeneration (5+) until the beginning of the end friendly magic phase.
The sorceror can choose to have this target all friendly units within 18", in which case the casting value is 14+.
Based on the other Lores the harder spells do not seem horribly unbalanced; without weeks of testing I cannot tell if it is perfectly balanced.
3. Plague Squall (8+)
Direct Damage. Place the large round template anywhere on the battlefield. It then scatters D6". All units (friend or foe) with at least one model under the template takes 3D6 S1 Hits, with no armour save allowed.
Once per battle, The wizard can choose to extend the range to the entire battlefield. If he does so, every unit takes 2D6 S1 hits, and the casting value is increased to 20+.
You have taken out the rule about units with Mark of Nurgle not being affected; if that was intentional then possibly lower casting costs as you will be hammering your own troops a lot.
5. Curse of the Leper (10+)
As is. Power up = 20+ Casting, unit takes 2 off all stats.
Is that 2 per turn or 2 in the first turn and 1 thereafter?
Warriors of Chaos: Lore of Tzeentch
Blessing of Eye (Lore Attribute)
Whenever a friendly model on the battlefield make a roll on the Eye of the Gods table, add +1 Power Dice to your next friendly magic phase.
Useful if you have one Tzeentch Sorcerer; however, it does not seem to benefit multiple Tzeentch casters. On the other hand one die per Tzeentch Sorcerer might be overpowered.
6 - Infernal Gateway
As is. Power Up = any double rolled in addition to the 11+ strength = wiped out unit, casting = 30+.
Given the lack of protection against this spell, I hesitate to comment on the balance; however, initially given the likelihood of failure on 8D6 ( above average Power Dice generation) the big cast might be limited enough.