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Following Cortana's supposed "death" after defeating Ur-Didact, she was instead sent into slipspace and arrived at the Domain on Genesis. The Warden was intrigued with Cortana, believing that she would uphold the Mantle of Responsibility, admitting that were she not intelligent, he would have burned her from the Domain.
In 2558, during the attack on Meridian Station, the Warden Eternal was protecting a Guardian that John-117 and the rest of Blue Team were attempting to board. When Fireteam Osiris attempted to pursue them, the Warden Eternal appeared before them. He explained that he stood in service to Cortana and that Osiris were not permitted on the Guardian before he incapacitated them with his hard light sword. Osiris was able to defeat the Warden.
The Warden appeared to the Master Chief and Blue Team on Genesis, where he discussed John's relationship with Cortana. When the Master Chief ordered the Warden to take him to Cortana, the Warden retreated, telling him that it was not yet the time. Instead, he set the local Prometheans on Blue Team, knowing that he would threaten Cortana's rule. The Warden explained that Cortana and the other Created would inherit the Mantle of Responsibility. The Warden eventually entered the fray himself, but he was defeated by Blue Team.
During the Battle of Sunaion, the Warden attacked Fireteam Osiris in an attempt to stop them from boarding the Sunaion Guardian and going to Genesis, but the Fireteam defeated him and his Prometheans with the aid of Arbiter Thel 'Vadam.
The Warden spawned one and then another two additional bodies to prevent Osiris from linking up with Blue Team when they arrived on Genesis, but he was unsuccessful both times. After Cortana took Blue Team away from Osiris, this put the Warden "past all restraint", according to 031 Exuberant Witness. The Warden continued to deploy Prometheans and spare bodies of himself to stop Blue Team, but was unsuccessful.
Cortana berated the Warden after reducing the number of bodies that Blue Team had to fight to three, asking if the Warden believed her to be too foolish to trust who she liked. The Warden continued to defend his actions, stating that John-117 and Blue Team intended to bring an end to her rule before she even started. Blue Team defeated the three bodies and continued on to meet with Cortana. By this time, the Warden generated even more bodies and surrounded Blue Team, nearly killing them as they were trapped on a ledge, but Cortana intervened once more, removing all but one of the bodies, which appeared to severely weaken him in the process. He lamented that he had failed Cortana, before she manifested herself and placed her hand on his shoulder, causing him to fade away. The Warden would be proven right however, when John-117 attempted to convince Cortana to stand down.
The Warden Eternal is fought several times during the campaign, often accompanied by Promethean enemies, save the final battle with the Warden where Blue Team fight three Warden bodies. The Warden himself is a powerful and extremely difficult foe. He can attack in a variety of ways: his hardlight sword is usually an instant-kill, and in addition, his likely close proximity to a target after killing them with his sword makes reviving them that much more difficult. He can also send out a shockwave with a small radius, fire a beam from his eyes that is powerful enough to deplete a Spartan's shields and in addition can stun vehicles with an EMP effect, and use a charged attack similar to a black hole that is an instant-kill long range attack. In addition, the Warden is heavily armored; however, his armor is weakest in the back. If he is attacked enough times, he can be temporarily stunned for a brief moment, allowing time to deal extra damage. It is also possible to assassinate the Warden, however, his health must be sufficiently low in order for you to do so.
The Warden Eternal can also appear as a Legendary or Mythic boss in Warzone; he is usually one of the last possible bosses to spawn during a complete match of Warzone, usually around the 18-minute mark.
- Halo 5: Guardians (First appearance)