Wardens were a type of Swarm Scion that wielded dual Breaker Maces, made from the Locust crystal shells. The Wardens were vanguards of the Swarm, and their presence was often a sign that the Swarm was planning to turn a location into a new Hive.
While most Scions had shed a majority of their impenetrable chrysalis, Wardens still retained most of their chrysalis shells, making them almost completely immune to gunfire - except for their heads, which were the only parts of their bodies that had shed the crystal shells. As a result, Wardens typically wear bullet-resistant helmets. Fortunately, the helmets can be knocked off by bullets and explosives.
Despite being almost impervious to ballistic damage - save for their vulnerable heads - Wardens are vulnerable to explosives such as Frag Grenades and Boomshots, shock damage from a Shock Grenade, fire from an Incendiary Grenade, the blinding effects of a Flashbang, the freezing effect from Cryo Cannons, and the blunt force of a Breaker Mace.
Siege of Riftworm Village
One Warden was seen in the last moments in the siege of Riftworm Village attacking several Outsiders. He was possessed by Kait when she was connected to the Swarm Hivemind through a Snatcher and was used to attempt to protect the Village Chief Oscar Diaz and the remaining Outsiders. However, this fails when the Warden snaps out of her control and executes Oscar with Kait helplessly watching through his eyes. The Warden was presumably killed or retreated when the Swarm assault forces had been vanquished.
Return to New Hope
After Kait, Del and JACK arrived at the New Hope Research Facility to find evidence of Kait's visions, the pair encountered a Warden outside of the main building which engaged the three in an intense battle. The Warden was killed after being shot in the head multiple times. Kait and Del realized that the Warden's presence meant that the Swarm was scouting out New Hope as a potential Hive.
When Delta embarked upon a mission to launch the UIR's Hammer of Dawn satellites at OZP-11, they encountered another Warden in the UIR Rocket Hangar supported by heavy Drone and Juvie forces. The Gears managed to kill the Warden and his forces and continue on.
At the Train Turntable, a Warden engaged Delta aided by Juvies and a Bastion after Delta defeated several Drones and two Scions. Delta was able to kill the Warden and send the Rocket Transport Train to the hangar.
Battle of Old Ephyra
During the Battle of Old Ephyra, Delta Squad encountered a Warden in a ruined building in East Barricade Academy while trying to deploy a Hammer of Dawn targeting beacon. Delta killed the Warden and deployed the Hammer of Dawn to kill a nearby Swarmak and Swarm reinforcements. Continuing, they found evidence that the Swarm was trying to turn the whole area into a Hive, explaining the Warden's presence.
Hivebust of Sanctum
At the end of the Hivebust of Sanctum, Team Scorpio encountered a Warden who proved immune to the Venom Bomb that they had deployed in the Sanctum Hive. Alongside a Snatcher that was also immune, the Warden attacked Scorpio who escaped by riding Sanctum's door down a river of lava. The Warden watched them depart from the doorway.
Mission to Weilehi
On Weilehi, Team Scorpio fought a Warden amongst a tree with feathers taken from the Wakaatu which was releasing the Wakaatu's venom into the air. Scorpio managed to kill the Warden despite the disorientating venom and continued on.
Mission to the Wakaatu Nest
On their way to the Wakaatu Nest, Team Scorpio encountered Swarm forces and uncorrupted DeeBees from a nearby crashed Vulture battling each other. Amongst the Swarm forces was a Warden, but the combined forces of Scorpio and the DeeBees killed it.
Hivebust of Bravelle
During the Hivebust of Bravelle, Team Scorpio encountered a Warden in the vehicle bay on the surface of the bunker holding the Swarm Hive. Although Scorpio quickly opened fire, they were unable to harm the Warden who struck at them with his Breaker Maces, sending Keegan and Mac flying. The Warden wounded Mac's leg and Keegan's chest when Keegan attempted to come to Mac's aid. As the Warden prepared to kill Mac, Lahni jumped on him from behind and stabbed the Warden in the neck with her Combat Knife, wounding him. Enraged, the Warden grabbed Lahni by the throat and held her in front of him as Keengan opened fire with a Snub Pistol and Lahni kept stabbing at the Warden.
Getting an idea, Mac ordered Keegan to keep the Warden busy and as the Swarm creature strangled Lahni, Keegan managed to shoot off the Warden's helmet and wound him in the mouth. Enraged, the Warden released Lahni and charged Keegan who continued firing and ordered her to get out of there. As the Warden got face to face with Keegan, he ran out of ammunition and was about to be killed by the Warden. However, Mac drove one of the Rat Trucks in the bay at the Warden, rammed him through the bunker wall, outside and into another wall below. Mac told the laughing Warden that he'd stolen Mac's life and that "his name was Dillon" before stabbing the Warden through the left eye through the truck's shattered window, killing him.