Suggested by Rookie
Because Tzeentch was bored a new threat now coming to the throne world of Palpatine and the location of his towering Imperial Citadel. The Kraken’s Legacy alliance of Dark Eldars and Eldars joined forces with their tyranids enemies on Valedor and later they joined forces with Necron’s World Engine in order to capture planet Byss. In 20 days they must have control over entire planet and also capture Imperial Citadel ( mostly intact). They start at the edge of the system. World Engine’s guns are not allowed to fire on the planet, but World Engine’s interceptors (and other air units) and land troops are allowed to join land siege. Eldars, Dark Eldars and Tyranids are allowed to use webway to teleport their troops on the planet, although Necrons troops can’t use this way.
Emperor Palpatine prepared for siege as well. Both World Devastators, Death Squadron, Entire Byss defence fleet, Eye of Palpatine, Lusankya, Eclipse II and Eclipse I of Eclipse-class dreadnoughts, Sovereign-class Super Star Destroyer, Mandalore the Ultimate with entire land army of Mandalore planet arived to help as well. Darth Malgus, Darth Sion and Darth Caedus also arrived to help Palpatine. Darth Vader, Count Dooku, general Grievous and Darth Maul also showed up to help. Byss also got a brand new army of 10 000 Phase I dark troopers, 5 000 of Phase II dark troopers and 800 of Phase III dark troopers. Palpatine can also summon land armies from Kuat and Coruscant (during Galactic Empire prime). All of Palpatine’s Inquisitorius arrived on Byss. And lastly they got battle group of Yun-Yuuzhan (under the direct command of Warmaster Tsavong Lah) as a gift from Vong‘s military forces. Although not all went good, galaxy gun was somehow destroyed just prior to the siege and now can’t be used.
Should Empire have any control of at least 1 city or Imperial Citadel at the end of the 20 days siege they will win.