From Halopedia, the Halo wiki
Project YGGDRASIL was the umbrella name for initiatives carried out by the Materials Group, aimed at re-engineering the strategically significant technologies and components of the MJOLNIR Powered Assault Armor for mass production and systems integration into new and existing combat vehicles of the United Nations Space Command. These efforts met with mixed success during the Human-Covenant War due to a lack of engineers and high tier manufacturing facilities. However, once the conflict ended, the full potential of YGGDRASIL was realized.
Products of Project YGGDRASIL include the HRUNTING/YGGDRASIL Mark I ADS, the HRUNTING/YGGDRASIL Mark II [D] armor system, and the HRUNTING/YGGDRASIL Mark IX Mantis, the latter of which utilizes features from the MJOLNIR Project such as the neural interface link, a compact fusion power plant, and a shield generator. Algolis was home to numerous facilities that housed prototype weapons of Project YGGDRASIL, including Weapons Research Facility T12A. During the Algolis Invasion, the UNSC was forced to destroy this facility and the HRUNTING/YGGDRASIL Mark I ADS located there to prevent the Covenant from discovering data of the Project YGGDRASIL.