Sol System - The Solar Fastness: There was a time when the wholesale invasion of the Throneworld of Terra was utterly unthinkable. With the opening of the Great Rift, that time has passed. The Sol System today boasts some of the most formidable fortifications in the galaxy, manned by determined warriors and shielded by technology and faith. Yet still the greatest lynchpin of its defence is the Adeptus Custodes.Were some courageous or foolhardy invader to approach the Sol System, they would find themselves assailed before they had even cleared the Mandeville Point -- that region of a star system's outer boundaries beyond which it is safe to translate from Warpspace to the real. The system's halo-belt plays host to massive star forts, garrisoned by dedicated regiments of Astra Militarum and empyrically tethered in place. The rest of the outer system is densely laced with thousand-mile-wide fields of void mines, prowling system monitors and huge, vacuum-hardened hunter-Servitors of terrifying aspect.Were the invader to overcome these hazards, they would find resistance stiffening the deeper they pushed into the system. Heavy naval patrols from the Battlefleet Solar thunder through the darkness, their craggy silhouettes presaging death to any who fall beneath their sights. Monitor-shrines, dock-fortresses, fighter bases and countless weapons platforms dot the darkness, their lumen winking like artificial constellations.The Grey Knights, the Inquisition and the Adeptus Mechanicus all have holdings within the Sol System, boasting suitably ferocious defences. Moreover, since Guilliman's Throneworld Decree, Terra and her sister worlds can raise greater forces in their own defence than ever before.When coupled with the efforts of the Solar Watch, a group within the Adeptus Custodes who have sworn binding oaths to keep guard over the Sol System, and the presence of the Imperial Fists' mighty mobile battle station Phalanx hanging in geostationary orbit above Terra, it is clear that the defences of the Sol System are all but impenetrable to invading armies. Yet there are subtler threats to the Golden Throne, and it is against these that the Adeptus Custodes must be especially vigilant.From all across the Imperium come endless streams of pilgrims, merchants, bureaucrats, adepts, zealots, emissaries, refugees and countless others. Thousands of voidships translate from the Warp every day, entering strictly coordinated approach corridors that lead them to Venus, Mars, Jupiter, Luna and Terra itself.Every world and moon in the Sol System -- barring a few mysterious exceptions -- is ringed with habitats and docking platforms, while all those whose surfaces are sufficiently solid play host to sprawling hab complexes, manufactoria and city-sized fortifications. An unending river of Humanity pours along the system's space lanes and fills its structures to capacity and beyond. It is amongst these masses that rebellion, sedition and heresy can and do foment.It is, of course, the duty of the Inquisition to root out such threats, and indeed some of the Holy Ordos' most zealous agents spend their entire lives doing just that. However, even these pious Imperial servants are not immune to corruption, whether by nihilistic ideologues, Chaos taint or xenofanatical mesmerism.Thus the Adeptus Custodes maintain their own presence, and perform their own patrols and monitoring sweeps throughout the Sol System. It is they who supervise the vetting of the countless Imperial citizens who pass through Terra's monolithic spaceports every solar day. It is they who enact surprise sweeps and inspections of the Sol System's defences, following no set pattern or schedule beyond their own intuition to prevent any from predicting and avoiding their vigilance.The Custodians seed listening devices, spy-servitors and dictalarcenous subroutines through the hives of the Throneworld and beyond. They gather and analyse every scrap of information they can from even the darkest reaches of the Sol System, feeding ship names, population movements, labourers' shift schedules, demagogues' sermons, vox intercepts and more into macro-cogitators the size of battleships.The data prophecies that emerge from these vast engines aid the Captain-General of the Adeptus Custodes in his command decisions on a daily basis, and help the Ten Thousand to be ever-vigilant.