[Blueberrypop]

Herald of Nurgle-90
-Breath of chaos
-Noxious touch

Beast of nurgle-35

Plaguebearers oh nurgle-155
-10
-Instrument of chaos

Plaguebearers oh nurgle-155
-10
-Instrument of chaos

Daemon prince of chaos-145
-Mark of tzeench
-Iron hide
-Soul devourer

Daemon prince of chaos-170
-Mark of tzeench
-Master of sorcery
-Bolt of tzeench
-Daemonic gaze

Mind you I have no idea what I'm doing

 

[AAAAAAAAARRRGGHH]

Herald of Nurgle-90
-Breath of chaos
-Noxious touch
This one sucks. Sure, it's some of the better things you can give this guy but nurgle heralds are awful. Really awful. Any other herald would be of much greater help in your list tbh.

Beast of nurgle-35
If there's any unit in the Daemons codex that can be considered useless; this is it. Think about it. You are paying 35 points for a unit that has a random number of attacks, low WS, low initiative, is slow and generally can't kill anything due to lack of power weapons. What is it good at? Not dying. But since it is not a scoring unit one wonders why this model was even given stats. Seriously, if you are bringing this piece of shit I ask you: Why even bother playing the game? Might as well do something worth your time. I am not exaggerating. It's that awful.

Plaguebearers oh nurgle-155
-10
-Instrument of chaos

Plaguebearers oh nurgle-155
-10
-Instrument of chaos
Plaguebearers, on the other hand, are awesome. Why? Because they are scoring. And extremely hard to remove from an objective sitting in cover. They are so tough that you actually are wasting points by making units of them any bigger than 7!
Remember, all they can ever do is hold objectives (occasionally you will find units that they actually can beat in combat. Like fire warriors) so giving them stuff like the instrument is unecessary. So keep them cheap. That way you have more points to use on units that actually kills stuff.

Daemon prince of chaos-145
-Mark of tzeench
-Iron hide
-Soul devourer
Why would you ever give a prince soul devourer? He already ignores armour and the instagib ability is pretty redundant. Usually princes rape in H2H. And if they don't, a random instagib effect is not going to change that. Give him bolt instead. That way he can actually shoot tanks. Something which daemons really need. Tbh though, at this points level you should buy more troops. Princes quickly take up way too many points.

Daemon prince of chaos-170
-Mark of tzeench
-Master of sorcery
-Bolt of tzeench
-Daemonic gaze
Better, but Master of Sorcery is pointless as MC's can already fire 2 weapons a turn. And you should probably get more troops rather than princes at this points level.
Mind you I have no idea what I'm doing

I can see that. Take a look at some other lists for inspiration because, clearly you need more experience with this army. Like with any army there's plenty of do's and don'ts with daemons, but the don'ts are just even more important to avoid. Bringing nurgle beasts and heralds is one of them.

Btw, I love your avatar. Lol

 

[Blueberrypop]

Herald of khorne-100
-Iron hide
-Unholy might

Fiends of slaanesh-160
-Unholy might
-5

Plaguebearers of nurgle-75
-5

Plaguebearers of nurgle-90
-6

Bloodletters of khorne-160
-10

Daemon prince of chaos-160
-Mark of tzeench
-Bolt of tzeench
-Daemonic gaze

Any better?

Herald with his entourage of bloodletters. Fiends to go mess people up or hopefully come in behind units(probably going to be split into two units of 3). PB to just be tough lol and the prince of pewpew to HOPEFULLY pop a tank if need be.

 

[AAAAAAAAARRRGGHH]

Much better!
Fiends shouldn't be in units below 5. Too easy to kill then. They should also have UM just to make them a tad more survivable. Otherwise a pretty good list. My only aber dabei is the herald. I prefer Tzeentch or Slaanesh ones in chariots as they are just too good to pass up. Fast, reliable and good at many things. But at least the khorne ones kills things. Unlike those awful nurgle ones.

I would still leave the princes out of games beneath 1250 points. But it's all preference.