Welcome jordster100 to Heresy in case nobody has said it yet! I noticed your thread and decided to take a peek at it. Please don't take anything I say below to being insulting or overly critical of your chapter. I see the general theme you have for your Chapter and the vision (I think) you had while forming them.
The Spartans Space Marine Chapter
Founding Chapter: Iron Hands
Founding: Seventh Founding
Chapter Master: Helican Elric
Homeworld: Jaeger (Planet) , Amestria (Moon)
Fortress-Monastery: Rhinorium City
Colours: Black with dark red shoulder pauldrons and a knee pad
Specialty: Ranged and Siege warfare
Strength: 444 excluding dreadnoughts
The Chapter name seems a little off to me, being blatantly called the Spartans. This harkens back to a past that most of humanity doesn't fully remember or understand. Remember the references to the Grekans and the Romanii. I am not sure if the Spartans were remembered fully or not, but I would wager that they'd be remembered but not as the Spartans specifically. I understand the general idea behind the name but it may serve your Chapter better to change things up a bit. Giving them a name that is related to the Spartans, but not actually called them, may add some appeal.
I do like that you chose the Iron Hands, in many DIY Chapters I don't see a whole lot of love for the Iron Tenth. The color scheme also is appealing, black and red naturally mix well together.
The Spartans, were a chapter created in the seventh founding by the Iron hands space marine chapter. The new legion was given the Terran world of Jaeger and its Moon Amestria by there father legion. Being descendants of the Iron hands legion their gene seed is from Ferrus Mannus. They were created by the Iron hands in order to combat Chaos forces near the eye of terror. Due to there proximity to the eye of terror, there is often chaos raiding parties made to their system which has lead to much of the chapter being destroyed. The thousand suns chapter has attacked the home world of the Spartans causing much of the chapter to be destroyed (Specifics in the Attack on Jaeger). After the last attack the system was fortified with a guard Regiment on both the Planet and the Moon of the System.
Adeptus Astartes Chapters can only be created with the express permission of the High Lords of Terra and then their "construction" if you will is only done by the Adeptus Mechanicus, and then usually under the supervision of some sort by the High Lords and maybe Inquisition. The Spartans may come form the gene-seed of the Iron Hands but the Iron Hands would not have the power to order or conduct their creation.
This will also roll over to reasons for existing in that the High Lords usually pick that for them, but then this often changes because the Adeptus Astartes are highly independent or orders just change. If you want them near the Eye of Terror, that's no problem, just say that the High Lords tasked them with garrison duty near the eye. And that the moon of Amestria was tagged by the Adeptus Mechanicus has a suitable recruitment world for Space Marines.
This is just an idea, so feel free to abuse it however you want, but perhaps the planet of Jaeger was chosen because it was somehow vital to its sector? The High Lords couldn't afford anymore Chaos raiding activity to disrupt the world and so permitted the Spartans control of the Amestria (the moon) and then nominal control over the planet itself? Just some ideas. As the Imperium naturally decayed during the Age of Waning and then coming up to the End Times I believe its been stated that the High Lords gave more and more planets over to the control of the Space Marines in order to ensure stability.
I say all of that because the Iron Hands really wouldn't have "given" the Spartans these worlds since the Iron Hands can only really lay claim to Medusa. These worlds would have to be allotted to the Chapter via the High Lords.
Also, its nice to see that the you've chosen the Thousand Sons are your enemy. They get so little love sometimes.
Although relatively small in numbers after the war with the Thousand sons, The Spartans have attained a single war hound titan during there campaign against Chaos but have not shared it with the mechanicus, to allow the Chapter to protect there home world from the threat of Chaos
This would be completely impossible to achieve. Warhound Titans are operated by specialist personnel that only the Collegia Titanicus has access to or can train. These individuals would not forsaken their Legio to join with any Astartes Chapter on a permanent basis, nor would the Mechanicus permit the Legios from permitting their divine god-machines to be "given" to anyone.
Even if the Chapter was able to convince the crew of a Warhound Titan to join them, their own Techmarines may rebel at the idea. This would be tantamount to blasphemy to them since its basically theft of a god-machine. They'd likely report you to the Mechanicus. Though, even if none of that happened, there are tech-priests everywhere and the Chapter relies upon the Mechanicus for supplies and resources. The secret Warhound wouldn't stay secret for long, they'd eventually find out.
Once the Mechanicus found out that the Spartans were holding a god-machine for their own purposes, it would be a disaster for the Chapter. In the very best case scenario the Mechanicus would raise nine-kinds of hell, inform the Inquisition and the High Lords and force the Chapter to return their god-machine. In the worst case scenario the Mechanicus wouldn't waste time with diplomacy and send the Legio that Warhound belonged to, to the Chapter homeworld and it would spark a major incident between the Adeptus Astartes and Adeptus Mechanicus. One that the Spartans would surely lose.
This loss goes well beyond the loss of face. The High Lords could sanction the Chapter for heresy, which could lead to Penance Crusaders, the forfeiting of their homeworld, or possibly being branded as Traitoris and summarily destroyed. If the Chapter managed to live through the debacle the Mechanicus would almost assuredly cease all support of the Chapter. This would severely weakened the Spartan's ability to resupply or maintain any kind of war footing. It would mean that they wouldn't train Tech-Marines for you anymore and the Chapter would gradually decay.
You could, possibly, reword this to say that your Chapter recovered a Warhound Titan that was once thought lost and reported it to the Mechanicum. In exchange, I'm sure they'd have given you some goodies (exotic weapons or wargear, but only a limited number) and the Legio the Warhound belonged to may have extended to the Chapter a promise of support of the defense of their homeworld in the future. This way you get to be the good guy and in any sort of major conflict in defense of Jaegar you would get more than just one Titan supporting you.
Attack on Jaeger:
A warp gate was opened on the Jaeger by Chaos cultists and Heritics allowing Deamons and Chaos space marines through. Specifically deamons of Tzeench attacked the Planet for months. The portal was far away from the Monastery and so very few marines were near the portal and so unable to effectively combat the Chaos threat. Upon learning of the attack Helican Elric ordered that Marines be withdrawn back to the planet from the war with chaos. After weeks of fighting Chaos forces had taken most of the planet and were on the doorstep of the monastery. If by fate or luck the Spartans crusading forces arrived back at the planet and fought back against Chaos. It was at this time when an unlikely ally showed up the Eldar of Ulthwe appeared from nowhere and attack the Chaos forces. Neither knowing or caring why the Eldar supported the space marines The combined forces were able to push back the Tzeench forces although at a cost to the Spartans. 3 Entire companies had been wiped out and only 444 marines remained alive with over 600 dead. After the initial recovery, the Chapter began to recruit again to attempt to bring up there numbers however by the end of the 41st millennium The chapter is still at low strength and is rebuilding its home world.
I'm a little iffy on this myself. That a Warp Gate could openly suddenly on a Chapter home world is a little odd to me. Given the randomness of the Warp and the Ruinous Gods I'm not saying its outside the realm of possibility but in most cases the opening of a portal or gate to the Warp often requires a ritual of sorts.
That a ritual was done would require cultists and thus would mean that the local natives of Amestria were involved in Chaos Worship. I would imagine that during the screen process of initiates at least some taint of the warp would have been discovered by your Libarius and then a major effort would have been made to discover these cults, their practices, leaders, and so forth with a purge being quickly conducted.
Even if the cultists were never selected into the Chapter the opening of a warp portal would make major "waves" in the Immaterium as it began to coalesce and the veil between Mateirium and Immaterium would begin to thin. This would almost certainly catch the attention of a Librarian and would undoubtedly had ensured some sort of reaction by the Chapter.
Now this is another idea and maybe this was what you meant to imply while writing this. Perhaps you could link the Thousand Sons and the warp-gate together and say that the Thousand Sons infiltrated the planet. This would be believable since its the Thousand Sons, the greatest Chaos sorcerers I'm aware of, they could hide their presence and maybe a strong enough cabal could conceal (for a time) the portal's formation.