The Warrior Unit was a special subdivision of the Marleyan military, composed of Subjects of Ymir under Marley's control who were given the power of the Titans.
History: In the early 830s, the Marleyan government announced the Warrior program to the Eldian populations of the country, in order to train Subjects of Ymir from their early ages so that they could inherit the different Titans owned by Marley in order to infiltrate Paradis Island in the near future, with the objective of obtaining the Founding Titan, resulting in Marley conquering the entire island and acquire its natural resources, thus consolidating the country's status as the undisputed global superpower. Grisha Yeager and Dina Fritz, then members of Liberio's Eldian Restorationists, a group desiring to overthrow Marley and bring Eldia back to power, decided send their son Zeke, who has royal blood from his mother's side, to become one of the Marleyan Warriors, with the objective for him to prevent the Founding Titan from falling Marley's hands. Zeke then became a Warrior candidate, but had many difficulties in training, leading to him being emotionally distraught by the pressure exerted from his father and the constant harsh critics from his Marleyan instructor Theo Magath.
Story: After the failure of the Paradis Island Operation, the mission was postponed due to multiple nations declaring war upon Marley after allying to form the Mid-East Alliance, resulting from the loss of the Female and Colossus Titans being made public. Because of the nature of the conflict, which was mostly fought on the seas, Marley's Titans and Warriors didn't have much opportunity to use their powers to the fullest of their capabilities. The situation changed during the final battle of the conflict, at Fort Slava, where the Warriors managed to help Marley win the war.
Trivia: "Warrior" has occasionally been mistranslated as "soldier" in the official English release. " However, this is likely due to the fact that the significance of the Warriors was unknown at the time of Volume 8's publication, with the first distinction between "soldier" and "Warrior" not being made until Chapter 39 in Volume 10.
Source: Warrior Unit, available under CC BY-SA 3.0.