1800 points Nurgle and Tzeentch

Hi man. I'll try to give some advice without changing your list.
First of all, i think that at 1800, you have too few bodies. I'm not a fan of AV 12 walkers...especially because the two helbrutes are have only a 24" range, but still, let's see

KeXun said:

* Daemon Prince (335pts)
(Daemon, Fearless, Veterans of the Long War)
Gift of mutation, Ichor blood, 3x Mastery Level (become Psyker), Nurgle, The Black Mace, Wings


* Sorcerer (165pts)
(Champion of Chaos, Independent Character, Psyker)
2x Additional Mastery Level, Aura of dark glory, Gift of mutation, Mark of Tzeentch
* Power Armour
Force Weapon, Plasma Pistol

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In this two guys, imo, you are dumping too many points. Also, i'd consider using the Spell familiar. Under the new psy rules its just mandatory for any psyker. Rerolling psy tests? all day long! Drop gift of moot-tation (pun intended) and ichor blood and get the familiar. Drop the plasma pistol and get the familiar.

KeXun said:

* Chaos Space Marines (225pts)
9x Chaos Space Marine, Heavy Bolter, Mark of Nurgle, Plasma Gun
* Aspiring Champion
(Champion of Chaos)
Bolt Pistol, Plasma Pistol, Power Weapon

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You are mixmatching weapons, wich can be fun and fine, but you should give them a common purpose. I'd say: plasmagun, autocannon, combiplasma (champ). This way you stay to Str 7 all along, the champ spare 5 points, go to 1 shot only, true, but has 24" range and a shot more in 12" range. Plasma pistol is just too short ranged to be used more than once in a game, imo.

KeXun said:

* Thousand Sons (Troops) (327pts)
(Fearless, Mark of Tzeentch, Slow and Purposeful, Veterans of the Long War)
9x Thousand Son
* Aspiring Sorcerer
Gift of Mutation, Melta Bombs
* Chaos Rhino
(Repair)
Combi-bolter, Havoc Launcher, Searchlight, Smoke launchers

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I'll drop a model to make space to the HQ sorceror and i'll also drop the moot-ation again and the havoc launcher. Since you are going to get midtable with this guys you might want to give them something that they lack: anti tank. namely a combi melta. or a combifamer if you plan fighting hordes.

KeXun said:

* Raptors (135pts)
(Fear)
Mark of Nurgle, Meltagun, Plasma Gun, 4x Raptor
* Raptor Champion
Bolt Pistol, CCW

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Again mixmatching weapons so different is never a good idea. I'd just give them 2 plasma guns or 2 flamers, if you expect hordes

KeXun said:

* Defiler (230pts)
(Daemon, Daemonforge, Fleet, It Will Not Die)
Daemonic possession, Dirge Caster, Power Scourge, Reaper Autocannon, Warpflame gargoyles

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This guy...what to say...i hate defilers. so many points for so frail a unit and such low fire output. You could swap him for a Hades Forgefiend and 10 cultists / 3 more nurgle marines

KeXun said:

* Helbrute (100pts)
(Crazed)


* Helbrute (100pts)
(Crazed)

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They can be ok, but you must use them wisely. If it was up to me i'd drop one of them and use the Helcult formation: 1 helbrute and 2 cultist units, fearless and granting the brute a hefty 3+ cover.

I still feel the army is too frail. My take with the same "feeling" would be:

Daemon Prince, Nurgle, Wings, 2nd Level, Spell Familiar (265) Just swoops around summoning Daemons and sweeping over enemy flyers (d3 Str 6, AP 2 hits...)

Sorceror, Terminator armor, 2nd level, MoTzeentch, spell familiar (140) Joins the TSons and buff them, maybe with telepathy?

15 Marines, nurgle, 2 plasmaguns, combiplasma (290 pts)

8 TSons
1 Aspiring Sorc,
Rhino, combimelta (264)

Helcult Formation: (240 pts)
Helbrute
20 cultists
10 cultists

Heldrake, baleflamer (170)

5 Raptors, nurgle, 2 meltagun (130)

Maulerfiend (125)
Forgefiend (175)

-1801-

So, lots of bodies, same number of daemonic walkers, still low on antitank, but still can defend against a non-spam mech list, some defence vs aircraft and more optimized psychik phase
Hope it helps, cheers!