The '''Fourth Tyrannic War''' began in [[M42]], when the largest invasion of Leviathan yet when it attacked the [[Western Reaches]] of [[Segmentum Pacificus]].{{Fn|21}} The assault began with the coordinated attack from above and below the galactic plain by two new tendrils dubbed [[Nautilon]] and [[Promethor]] respectively. The two tendrils are moving in parallel towards [[Segmentum Solar]], and with the [[Imperium]] distracted by the [[Indomitus Crusade]] few forces initially stood against it.{{Fn|24a}}
When ====Discovery====The first indication of the Tyranid thread was silence across much of Segmentum Pacificus as the [[Shadow in the Warp]] descended. The loss of communication with many outlying [[System]]s initially went unnoticed several [[Sector]] commands due to the typically slow wheels of Imperial bureaucracy, the true extant presence of [[Warp Storm]]s, as well as simultaneous [[Ork]] and [[Knydd]] threats. The scale of the threat the tendrils was posed was finally discoveredin the [[Morpheum Sector]] command thanks to the horrific death of High [[Astropath]] [[Guideoth]]. Consumed by the Shadow in the Warp, the Astropath was transformed into a tendrilled monstrosity that wrought destruction across the [[ImperiumAstropathic Choir]] before finally being put down. Simultaneously, reports finally began reaching Imperial authorities of a massive mobilizing Tyranid attack of unprecedented scale. The [[Byroth Sub-sector]] was consumed by Tyranids as its [[Imperial Guard]] forces battled Orks. At the [[Trinary Reach Cluster]], a [[Leagues of Votann]] fleet suddenly retreated as the Imperial defenders of several factory worlds rejoiced, only for the Tyranids to stop themdescend in their wake. Vast refugee fleets began arriving in the [[Haratus Stars]], the [[Mytolian Belt]], [[Templum II]], and [[Gethsemanax]]. However this would take timeDespite security precautions and quarantines, as Tyranid xenoforms and spores still got through and caused [[Genestealer Cult|bloody uprisings]] in the vast majority aftermath.{{Fn|24a}} Gradually, Imperial commanders were able to put together a horrifying picture. Two new tendrils of Hive Fleet Leviathan of unprecedented size had invaded Imperial space in a coordinated effort. One, dubbed [[Hive Fleet Nautilon|Nautilon]], was moving into the [[Vynor Sector]] from above the galactic plane while the forces sent other, dubbed [[Hive Fleet Promethor|Promethor]], was moving into the [[Morpheum Sector]] from below it. Worse still, these two tendrils were coordinating with one another in a roughly paralleled eastern advance. If was a route that, if unaltered, would be take them into [[Segmentum Solar]] and towards Holy [[Terra]] itself. The Imperial response to this horror was typically inconsistent, with some [[Governor]]s oversee total mobilization to conduct a defense while others simply led a mass abandonment. Some [[Space Marine]] and [[Imperial Navy]] elements sought to wage whatever defense they could while other [[Astra Militarum]] armies began to attempt to create defensive lines to stem the Xenos advance.{{Fn|24a}} ====Imperial Response====Soon enough, the [[RegimentsEyes of the Emperor]]of the [[Adeptus Custodes]] learned of the Tyranid threat and the danger it posed to Terra. The Imperium[[Frigate]] ''Pinion''s more elite armies could attack docked at once though Terra shortly after, bringing word of the threat to[[Captain-General]] [[Trajann Valoris]] himself. Valoris subsequently met with the rest of the [[High Lords of Terra]]. With the forces of the Imperium mostly committed to the [[Indomitus Crusade]], many feared that Segmentum Solar was now defenseless to face the incoming threat. However Valoris and most [[Lord Commander Solar]] [[Arcadian Leontus]] oversaw a massive mobilization effort to ensure that the Tyranid would be blunted. A policy of these "aggressive defense" was formulated by the garrisons of the [[Imperial Guard]] while rapid-response strike forces were organized into known as [[Solblade]]s, which were named after their commandersestablished for more elite forces. These consisted Solblades would consist of a mix of not only [[Space Marine]]s but also , [[Sisters of Battle]]s, [[Militarum Tempestus]], [[Knight]]s, [[Mechanicum]], [[Inquisition]], and even [[Adeptus Custodes]]and were given a wide range of strategic autonomy. It was hoped that while they were low in manpower, their highly mobile and autonomous nature could disrupt the Tyranid advance. However By the time the first Solblades appeared, over half-a-dozen [[Lord Commander Sector]]s in Pacificus had been consumed by the Tyranids, but proved highly capable in disrupting the Xenos' progress in local counterattacks.{{Fn|24a}} As the Solblades bought time, Valoris and Leontus oversaw the establishment of a defensive line to protect Segmentum Solardubbed the [[Sanctus Line]] . This was based around key "[[LeontusAnchor World]] also established s", each of which lay on a major warp transit route. While the Tyranids did not use Warp travel, these worlds were still of vital importance logistically to the Imperium and it was hoped that using them as the lynchpin in the defensive network would allow for a unified and coordinated front against the incoming Tyranids. Each Anchor World was given a [[Anchor WorldsSub-Sector]]s worth of resources, and in case addition to their defensive role they also provided bases for Solblades to operate from.{{Fn|24a}} Deploying into the Solbades could not stop paths of the tendrilsof Promethor and Nautilon, the Solblades found themselves facing massive concentrations of Tyranids across a large multi-system front. They were The tendrils did not only heavily fortifiedmove swiftly however, and each comprised scores of smaller splinter fleets that broke away and sought out new prey. Sometimes, but also served as waypoints they would even double back to funnel strike at enclaves of Human or other alien life. The Tyranids were taking their time, and this was something the Imperium's forces sought to exploit. using their speed to their advantage, the Solblades emerged victorious in countless tactical engagements, though their own losses were also terrible. Gradually, under constant Imperial and resources into other alien attack, the invaded Segmentum Pacificusadvances of Nautilon and Promethor stalled and began to fragment. Hope would prove to be short-lived, however. {{Fn|24a}} ====Grendyllus====While the Solblades eventually proved successful in slowing [[Nautilon]] and [[Promethor]]'s advance, the [[Hive Mind]] countered this by launching its third tendril dubbed [[Grendyllus]] up through the galactic plane and then attacking well behind the lines of the Solblades' advance. This allowed it to rampage through several [[sectors]] and [[Leagues of Votann]] territory before it invaded the [[Bastior Sub-sector]]. It was only when [[Groupmaster]] [[Tysen Yacobe]]'s [[Battle Group Faustus (Fleet Sextus)|Battle Group Faustus]] encountered the Hive Fleettendril, that the Sub-sector became aware of the true extant of the invasion.{{Fn|24a}}
At that time, [[Lord Commander Solar]] [[Leontus]] was located in the [[Bastior Sub-sector|Sub-sector]], as he had made the [[Anchor World]] [[Sanctum]] his base of operations. When he realized the full scale of the invasion, Leontus knew Bastior's [[Imperial]] forces were not enough to defend it. The Lord Commander Solar would then send out a plea for aid to the [[Solblades]] and any other Imperial forces outside the Sub-sector that could help. Afterwards, Leontus ordered Bastior's Imperial forces to retreat from their current positions and reinforce either the [[Formidyre System|Formidyre]], [[Pyremoat System|Pyremoat]] or [[Kastayl Systems]], which were named as '''Sanctuary Systems'''. However this would not be possible for all of the Imperial forces, as they were already engaged with fighting [[Hive Fleet Grendyllus]] or were unable to reach the Sanctuary Systems. Others, like [[Groupmaster]] [[Tysen Yacobe|Yacobe]], meanwhile refused to retreat and either struck at the Hive Fleet whenever possible, or defended the worlds and [[System]]s they were located in when the invasion began. Some Imperials even attempted to flee from the Sub-sector, but only a few were successful{{Fn|24a}}. Species like the [[Leagues of Votann]], [[Orks]], [[Asuryani]], [[Drukhari]], [[Necrons]], [[Hrud]], [[Ghluthykk]] and [[Mebrii]], also either fought or fled from Grendyllus. Soon the entire Sub-sector was invaded and several of Bastior's systems fell to the Hive Fleet. Sanctum itself seemed to be targeted by Grendyllus and some Imperials in the Sub-sector speculated this was because the Hive Fleet knew of the Anchor World's importance.{{Fn|24b}}
====Known Campaigns of the Fourth Tyrannic War====
*Defense of the The [[Battle of Bastior Sub-sector]]'s - Key campaign against [[Formidyre SystemGrendyllus]]. Led by the [[White Templars]] and [[Imperial Fists]]{{Fn|23}}*Defense of the Bastior Sub-sector's [[Stanghalde System]] by the [[Raven Guard]] [[Shadow Captain]] [[Sard Gaeron]]{{Fn|24c24a}}
*War against the [[Mebrii]] and other [[Xenos]] species within the Bastior Sub-sector{{Fn|24b}}
*Invasion of the [[Sanctus Line]] world [[Regium]]{{Fn|25}}
*{{Endn|23}}: [https://www.warhammer-community.com/2023/06/05/warhammer-40000-global-battle-decide-the-fate-of-a-world-and-unlock-massive-reveals-in-the-battle-for-oghram/ Warhammer Community: Warhammer 40,000 Global Battle – Decide the Fate of a World and Unlock Massive Reveals in the Battle for Oghram (Last accessed on 05/06/2023)] (Last accessed on 05/06/2023)
*24: [[Leviathan (Box)]] Rulebook
**{{Endn|24a}}: pgs. 222-230**{{Endn|24b}}: pgs. 230-233 234**{{Endn|24b24c}}: pgs. 238-243**{{Endn|24c24d}}: pgs. 247-255
*{{Endn|25}}: Leviathan (Novel), Chapter 1
