Well, I normally don't do this as it tried my patience at the keyboard, but you seem eager to receive feedback, so here goes...
I will be blunt, and not everything I say is perfectly accurate, but I'll try and fix the glaring errors and help with the writing proper.
Ogryn wrote:
Founding: Thousand Sons Pre-Heresy successor chapter
There is no such thing as a pre-heresy chapter. Only the 18 legions existed before the heresy, and only the loyalist legions were split into chapters following the introduction of the Codex Astartes after the heresy. This needs to be changed. Your main options are to have them be some kind of third founding chapter (second founding chapters are all catalogued to the best of my knowledge, and third founding is still really early) that fell to Chaos/Tzeentch and became powerful psykers. The problem with using Thousand Sons as a base is that they no longer have bodies, they're simply souls trapped inside power armour from Ahriman's Rubric. I would honestly scrap any idea involving a chapter/warband that is directly descended/split off from the Thousand Sons as it makes no sense as per the fluff. I'd create a chapter that fell to Tzeentch, achieving essentially the same thing.
Ogryn wrote:
Armour: Golden, with various markings, and designs
Various markings? Designs? What kind? Be specific. Give your chapter life, make it different and unique. You might as well call your marines 'The Gold Marines'. "They do sorcery and stuff. Tzeentch is cool." Give your army some flair and put the effort into properly done (not overdone) descriptions.
Ogryn wrote:
The Golden Eagles are a Pre-Heresy Warband of the Thousand Sons.
Again, this doesn't make sense. The Thousand Sons were a complete, unified legion when the Rubric was created/unleashed, turning them to dust inside.
Ogryn wrote:
Founded before the great Horus Heresy, these powerful Sorcerers live to cause destruction, and death to the Imperium. They have destroyed whole worlds of the Imperium, and slaughtered many Imperial scum.
Again, change the fluff. Also, saying things like "They have destroyed whole worlds of the Imperium, and slaughtered many Imperial scum." is beyond stereotypical and should largely go without saying. Giving examples would help instead some blanket, generic statement that just comes off as almost being lazy.
Ogryn wrote:
Destruction of Giuerta
Giuerta was a strong Imperial world, close to Terra. The inhabitants of Giuerta mined valuable minerals, and produced many products for the Imperium. The Golden Eagles decided to strike, and take the world for themselves. The Sorcerer Ast'idus flew down, accompanied by several Veteran squads. They ran into a large fortress, containing many Space Marines, who guarded Giuerta. Ast'idus slew the first two guards silently and coldly, by ripping his Daemon weapon through them. He kicked away the bloody corpses, and descended further into the fortress. A squad of Tactical marines rushed out, and fired upon the Golden Eagles. Suffering few casualties, the Golden Eagles Veterans returned fire, and blew the Tactical marines away. They ventured into the control room, killed the Space Marines, and disabled the Imperium's communication. The Golden Eagles called in for backup, and the Golden Eagle's fleet descended. Destroying many buildings, and slaying the inhabitants, Giuerta was theirs.
The Golden Eagles have participated in many various raids, and battles, although most records have been lost....
Right, I'll deal with this paragraph as a whole. What kind of planet? How many people? What minerals? What products? How did Ast'idus 'fly' down? A large fortress manned by a dozen marines? From what chapter? How did they blow them away? How many casualties?
An entire planet, one seemingly as useful as it is to the Imperium, is not defended by a single fortress with a single astropathic communications array by a dozen or two marines. Where is the PDF? All the citizens just kinda shrugged and carried on, getting slaughtered peacefully?
Honestly, if you're going to tell some kind of story about them, you go all the way, or you don't. Either give us a very brief synopsis with minimal detail, or tell us the whole story in all its vivid and gory detail.
Ogryn wrote:
Known Characters of the Golden Eagles
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More often than not, character descriptions just end up coming off as heavy 'Mary-Sue' style characters that fails to achieve the desired result. I would either skip doing it all together, or giving us a good story about each so we have a snapshot of who they are.
Ogryn wrote:
*PURGED FROM RECORDS*
One of the first followers of Tzeentch, he bears a huge golden crown. Legend has it that he is the sole son of Tzeentch himself.
Yeah...the sole son of Tzeentch...since when did Tzeentch have a son? And his signature thing is a large gold crown? That's it?
Ogryn wrote:
Greater Daemon Hy'ustias
Having served Tzeentch ever since the Horus Heresy, Hy'ustias has slaughtered dozens of Imperial scum, and razed entire cities to ashes.
Sounds too Khorne-ish.
Ogryn wrote:
Chaos lord Kit'res
Kit'res is a towering Chaos Lord, spreading fear throughout entire galaxies. Wielding a huge giant Daemon weapon in the form of an axe, he enjoys decapitating his victims, and keeping their skulls as trophies.
Again too Khorne-ish.
So...yeah, those are my thoughts on the matter. Take it as you will, but I honestly mean no offense, and you did ask for more help. Hope I was of some service anyways.
Cheers, and good luck!