From Halopedia, the Halo wiki
The Draugr is a massive vehicle employed by the Covenant, featuring a gravity lift and heavy-duty energy shielding. Analogue to a Land Battleship. It is among one of the largest and most powerful land vehicles of the Covenant, with only the Harvester and Kraken surpassing it in scale.
The Draugr is employed by the Covenant as the centrepiece of their largest mobile field armies, where they are flanked by Scarabs for protection. They can house legions of combat troops inside their shield dome, and thus make a useful tool when the Covenant wish to completely annihilate a well-entrenched enemy.
The Draugr was initially concepted by Isaac Hannaford under the working name "Siege Engine" during the production of Halo: Reach. At the time, the vehicle was to be the centrepiece of a cut level later in the game, and can be seen in several pieces of concept art. Hannaford designed the vehicle based on the H-K Tank from Terminator.
The vehicle was later brought back and canonised in the Halo Encyclopedia (2022 edition).