As their name implies, the Horsemen of Apocalypse are usually bad news for the X-Men. Since the Horsemen of Apocalypse debuted in Louise and Walter Simonson's X-Factor #15, the members of the ever-changing group have been Apocalypse's most dedicated generals. From Archangel to Wolverine, more than a few X-Men have been drafted into Apocalypse's Horsemen to take on the traditional roles of Death, War, Pestilence, and Famine.
And as Marvel's new teaser for Jonathan Hickman, Tini Howard, and Pepe Larraz's upcoming X of Swords crossover suggests, the First Horsemen of Apocalypse seem set to become very important to the modern Marvel Universe.
While the X-Men and Apocalypse are usually bitter rivals, the ancient X-villain has been working closely with Marvel's mutant heroes on the island nation of Krakoa. Even though he has a position of influence on the Quiet Council of Krakoa, Apocalypse has quietly been pursuing his own plans involving mutant mysticism in Excalibur. Although it's not yet clear how the First Horsemen fit into those plans, Marvel has been quietly building to their debut for several months.
The First Horsemen of Apocalypse made their silent debut in Jonathan Hickman and Pepe Larraz's House of X #2, where they stood silently next to Apocalypse and Moira MacTaggert, who had allied herself with the ancient mutant in this timeline. After Moira and Apocalypse woke them up, these Horsemen fought in the Apocalypse War that saw many of this timeline's heroes die.
While the true identities of these Horsemen remain unrevealed, their designs all seem to be inspired heavily by the iconography of ancient Egypt, with one Horseman -- presumably the Horseman of Death -- modeled after Anubis, the Egyptian God of the Dead. Although the precise fates of these Horsemen are unknown, these designs also carried over into Marvel's main timeline.
Within the Marvel Universe, the fate of the First Horsemen was eluded to in a flashback in Jonathan Hickman, R.B. Silva, and Marte Gracia's Powers of X #4. When demonic monsters broke the ancient island Okkara into the islands Krakoa and Arakko, Apocalypse and the Horsemen stopped them from spreading out into the world.
Apocalypse eventually pushed the monsters into the chasm from which they escaped and sealed Arakko away, with the Four Horsemen sent to guard that land. While that action could have sealed their fate in a very literal sense, the specter of these Horsemen has already started looming over the modern Marvel Universe too.
In Jonathan Hickman, Dustin Weaver and Joe Sabino's aptly-named Marvel Comics #1000 story, "The First Horsemen," Apocalypse entered an ancient temple in the Libyan Desert. He said his subsequent Horsemen were poor replacements for his original Horsemen, and he beckoned them to rise again. Curiously, Apocalypse refers to the Horsemen as his "sons and daughters" on this page. While that seems like a mere turn of phrase at first glance, the next mention of the First Horsemen suggests that they may actually be Apocalypse's children.
In Jonathan Hickman, Leinil Yu, and Gerry Alanguilan 's X-Men #2, an island mysteriously appeared and began heading towards Krakoa. When Cyclops, Rachel Summers' Prestige and a young Cable arrive on the island, they encounter a High Summoner of Arakko, a resident of the island who can summon monstrous creatures to defend it.
As that issue reveals, the High Summoner is the son of Apocalypse's First Horseman of War. Shaped by a lifetime of battles, this nameless, ageless figure is a link between Arakko and Otherworld, the mystical realm where Apocalypse recently helped the Omega-Level mutant Jamie Braddock assume power. With Arakko's enemies starting to become overwhelming, the High Summoner was sent through a portal on this new island, the Arak Maw.
After the Arak Maw merged with Krakoa, the High Summoner sought out Apocalypse and identified himself as War's son. With this, Apocalypse embraced the High Summoner and said that he means to save the children of Krakoa and Arrako.
To put it mildly, Apocalypse has never been the most tender character, as his role in Krakoa's Crucible attests. However, he legitimately seems to show affection towards the High Summoner and the First Horsemen. Where Apocalypse has usually been a relentless engine of conquest and destruction, the First Horsemen still seem to hold a special place in Apocalypse's heart.
Outside of the promise of having the X-Men take up swords, the details of X of Swords remain largely unknown. However, this X-Men crossover is shaping up to be Apocalypse's most personal battle yet.