From Halopedia, the Halo wiki
The Type-55 Ultra-Heavy Siege Tower (T-55 UHST), colloquially known as the Kraken, is a Covenant excavation walker repurposed as a siege tower. It was used in combat by Jul 'Mdama's Covenant in late 2558.
Some human records indicate that the Kraken may have been encountered early in the war with the Covenant, but none survived to confirm it. Jul 'Mdama's Covenant secured several weapons caches belonging to the original hegemony, including a number of Krakens. Although they were at first used sparingly (as 'Mdama wished to avoid "tipping his hand"), the Covenant deployed Krakens en masse in October 2558.
During the Battle of Kamchatka, 'Mdama ordered a Kraken deployed to hold off Promethean forces that had rebelled against his control. However, the Promethean resistance forced the platform to fall back, leading to 'Mdama's death shortly thereafter.
'Mdama's remaining troops attacked the Vadam lands of Sanghelios with another Kraken. Though powerful, it was held off by Swords of Sanghelios Banshees until it was cornered in a narrow canyon. There, Spartan Fireteam Osiris boarded it using captured Forerunner Phaetons, navigated to the excavator's power core and then overloaded it before escaping the Kraken on hijacked Banshees. Without power, the Kraken fell to its demise in the chasm below.
The Kraken is an ultra-heavy siege tower, a massive Covenant sky fortress with three powerful grappler arms capable of physically crushing and moving mountain-sized objects. The shape of the Kraken is very similar to terrestrial squid. This, along with its massive proportions, has given rise to its UNSC nickname, after the creature from Norse mythology. The Kraken is composed of a large hollow structure that allows large aerial vehicles such as the Type-57 Phantom dropship to fly and even land inside it. Throughout the Kraken, there are several Shade turrets deployed for defense. The area above the hollow section is solid, capping it off. Below the hollow section there are several more Shade turrets and docking bays that can house Type-54 Banshees.
The core is housed in the bottom-most section, where the three limb-like mechanisms protrude. The core is susceptible to heavy ordnance and becomes explosively unstable after sustaining significant damage.
The Kraken is able to hover via a Forerunner gravitational core, which also serves as its power source, but can also support itself on its limbs if needed.
It is commonly held that these siege towers were originally based on a variant of Forerunner Sentinels used during the war with the Flood, having been discovered in a reliquary of mothballed ordnance.
The Kraken features several Type-26 Shades spaced across its decks. Each of these is operated individually.
Its limbs can also be used offensively and are capable of massive structural damage.
The Kraken was codenamed "Sentry Ship" during development of Halo 5: Guardians to reflect on its role as a watcher overseeing the battlefield.
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