Astoran

Astoran is an Adeptus Astartes homeworld of the Blood Serpents Space Marine Chapter. It is located in the Segmentum Obscurus fairly close to the Eye of Terror and suffers from the occasional severe Warp Storm. The Blood Serpents were created as a part of the 12th Founding sometime in M35, as the Imperium wanted to add another chapter near the Eye of Terror to help stave off potential Chaos attacks. Seeing the world of Astoran as an ideal defensive world due to its tunnel network allowing for knowledge of the world to let a small force defeat a much larger one, the High Lords of Terra chose to build the Chapter there. Since their inception, the Blood Serpents have stood eternal vigil, acting as a bulwark between the Imperium and the enemies of of Mankind.

Background
Astoran is a Death World whose surface is completely unlivable to normal humans due to the immense radiation caused by the planet being so close to its sun with very little atmosphere. Space Marines in Power Armour can only last for about an hour on the surface before they die, and Terminator Armour only protects them for 3 hours. The only exception to this is The Scar, a 100Km long twisting rocky canyon which somehow managed to escape the radiation covering the rest of the planet.

Due to the complete un-survivability of Astoran the Blood Serpents live beneath the surface in a maze of tunnels designed to confuse anyone trying to invade the planet, while allowing the Blood Serpents to outflank the attacker. The origin of the tunnels is unknown however the Blood Serpents have since arriving massively expanded the tunnel network both near the surface and deeper into the planet. There are only three entrances into the tunnels of Astoran, the primary entrance, which is large enough for dreadnoughts to enter but nothing bigger. The Scar entrance which is at the northern end of the Scar which is large enough for the bikes of the Blood Scars to move through, with little room to spare. The final entrance is the Vehicle bay, a massive elevator large enough to comfortably hold 20 Land Raiders simultaneously that rises up to a massive set of doors that slide out of the way as the elevator rises.

The only structure on the surface of Astoran, other than a door, is Terminus Reach, a small fortress that was built for one purpose. In the event Astoran is overrun completely the Archserpents would teleport there and try to hold out as long as possible until help arrived at which point whoever arrived as aid would use the teleportarium on their ship to move the entire base onto their hanger. This is possible because the entire base is constructed like a massive suit of Terminator armor expressly for this purpose. Asterog's primary motive for doing this is to protect the only piece of Astoranium that has been found by giving it to Karoth, and having him bring it to whoever shows up to help.

Astoranium
Astoran is the only known source of an unusual compound known as Astoranium which has some sort of Synaptic disruption effect on the Tyranids. This compound also appears to have an ability to destroy Tyranid creatures with a strong connection to the Hive Mind, likely connected to the Synaptic disruption effect. Despite repeated attempts to recreate Astoranium the Imperium cannot recreate it. As a result Asterog has got a large number of human miners digging deep into the planet in search of Astoranium.

The Miners of Astoran
Since the creation of the Blood Serpents Astoran has been mined out, initially before the Fall this task was given to the Scout Company with the purpose of expanding the tunnel network, and increase the defensibility of the world. While mining Asterog found the first, and so far only known, piece of Astoranium, which he only kept because he thought that it looked interesting, so he turned it into a pendant which he wares under his armor to this day.

Since the Fall, Asterog has pulled the Scout Company from the job of mining, preferring to use them in combat, and has outsourced the mining of Astoran to aspirants of the chapter. Additionally, Asterog has changed the focus of the mining from expanding the defensive network to searching for Astoranium with large open caverns in an attempt to not miss anything.