(Astonishing X-Men I#1 (fb) - BTS) - At an unrevealed point in time after Apocalypse's rise to power, the mutant known only as Rex joined En Sabah Nur's organization. Rising through the ranks, he eventually became the tyrant's right hand man. Rex assisted his master in running his expanding empire, not only keeping track of his many enemies but also his various operatives and underlings, including the four Horsemen.
(Astonishing X-Men I#1 (fb) - BTS) - For years Magneto and his X-Men were able to keep the location of their headquarters hidden. Rex devised a plan to track his master's enemies down. He reasoned that having an operative follow the X-Men's teleporter Blink through one of her portals would lead them to the base. With Apocalypse's blessing, he selected prelate Delgado for the task.
(Astonishing X-Men I#1 - BTS) - Delgado succeeded in following Blink and Sunfire home from a mission to Chicago, but he got stuck halfway through the portal. Before Blink killed him, he managed to partially relay his coordinates to Rex.
(Astonishing X-Men I#1) - Rex went down to Apocalypse's throne room to report on Delgado's progress. As he made his way through the blacked out hall, he tripped and landed next to a pile of skulls. Apocalypse told him he kept hundreds of thousands of human skulls next to him, claiming he found the aroma to be soothing. He then prompted the nervous, babbling Rex to get on with it. After Rex told him of Delgado's success in narrowing down the location of the X-Men's base, he was astounded to see a disturbing look on his master's face. Apocalypse assured him everything was fine: he was simply smiling.
(Astonishing X-Men I#2) - Apocalypse was furious when he received reports that Abyss had failed to stop the Human High Council's Sentinel fleet from evacuating humans. He killed the people who brought him the news, prompting Rex to stay hidden until Apocalypse called him out by name. He wanted some good news and demanded to know if Rex knew where the X-Men's base was yet. Fearing for his life, Rex was delighted when he received a timely report that cited its location as the Xavier Estate. He showed it to Apocalypse who ordered Rex to prepare his entourage, for he would be dealing with them personally.
(Amazing X-Men I#2) - Hoping to gain additional intel on the X-Men's base and its defenses, Rex interrogated the mutant operative Karma while Apocalypse and Sebastian Shaw looked on. The young woman claimed she knew nothing, but Rex was convinced she must have overheard things during her time working in Warren Worthington's elite social club Heaven. Rex even threatened to have the Shadow King ravage her mind to look for the information, but by then the prospect of further torture no longer amused Apocalypse. He announced they would be moving in on the X-Men's base to take the fight to Magneto once and for all.
(Factor-X I#3) - When Apocalypse received word of Mr. Sinister's defection, he angrily called for Rex. He told his underling to send a message to the new commander of the Pens, prelate Alex Summers: the only reason Apocalypse agreed to have breeding pens in the first place was to indulge Sinister. With Essex gone, Apocalypse ordered their immediate shutdown, he also wanted all the prisoners culled.
(Astonishing X-Men I#4) - Magneto was taken to Apocalypse's citadel. There, with his assistant Rex looking on, En Sabah Nur began to torture his opponent, showing him glimpses of his wife who was still on a mission to stop Holocaust. Magneto proved defiant and tried to strangle his foe, but Apocalypse happily reminded him of all his recent victories: from capturing him and Bishop to crippling the Human High Council's Eurasian resistance.
(X-Man I#4) - Apocalypse was furious when Shadow King reported that Nate Grey was still alive. He vowed to deal with the young telepath himself, but first he focused on personally overseeing the torture of Magneto.
(X-Man I#4 - BTS) - X-Man tried to reestablish his psi-link with Magneto, which led to him having a vision about Magnus' torture at the hands of Apocalypse. He immediately flew off to New York to free the master of magnetism.
(Weapon X I#4) -
Apocalypse took a break from torturing Magneto, allowing Rex to update
him on the latest intelligence reports. Rex told him the Human High
Council's airship armada had left London, but bad weather events over
the Atlantic made it hard for the fleet of bombers to cross the ocean.
Rex followed his master down to the labs while he informed him about
Donald Pierce and Mikhail Rasputin's sketchy activities. He was
surprised to hear Apocalypse didn't care to take any precautions, En
Sabah Nur felt that all the players were in place for the endgame. He
then assured Rex that Pierce and his band of altered humans were more
adaptable than you might think. To prove his point, he showed his
startled assistant another experiment from the vats that birthed Pierce
and his ilk: a monstrous techno-organic hybrid of a two humans and a
plasma pistol trying to become a single being. The startled Rex
commented that ugliness was not a power.
(X-Men Omega I#1) - Apocalypse
and Rex watched Holocaust torture Magneto, noting that the master of
magnetism's resolve was weakening. Rex was then asked to confirm that
the trap had been set. Magneto boastfully proclaimed that the X-Men
would continue without him, but Apocalypse then calmly told him he was
aware of Magnus' masterplan because the Madri had read Bishop's mind. He
then presented him with the M'Kraan crystal that had recently been
delivered to him by Guido, his double agent within the X-Ternals. Before
knocking Magneto out, Apocalypse told him the X-Men would be coming for
the crystal in an attempt to wipe out this timeline and he was ready for
them.(X-Men Omega I#1 - BTS) - Warren Worthington sacrificed his life to blow up the forcefield generator protecting Apocalypse's citadel. As a result Nate Grey and the X-Men were free to enter the base while Prelate Summers and Jean Grey had a chance to evacuate Sinister's test subjects from the pens underneath En Sabah Nur's headquarters. Without the forcefield, Apocalypse's citadel was vulnerable to the Human High Council's impending bombing raid.
(X-Men Omega I#1) - Magneto regained consciousness in time to hear Rex confirm the Shadow King's shocked announcement that the Human High Council had managed to sneak their atomic bombs past the Atlantic Defense wall. As a result, the entire midwest was now a radioactive crater. Rex was ordered to expand the Sea Wall defense grid across the Atlantic so it would become an offensive weapon. Apocalypse would not hear his warnings about the possible consequences.
(X-Men Omega I#1 - BTS) - Rex made himself scarce during the final confrontation between the X-Men and Apocalypse and his forces which ended in Magneto killing his master.