The X-Men's haven of Krakoa is filled with powerful mutant family trees like the Summers, the Guthries, the Rasputins, and Wolverine's long line of descendants. While all of these families is wild in their own ways, they're practically tame when compared to the siblings of the St. Croix family, who made their debut in the first volume of Generation X.
The confusing St. Croix family history involves mutant vampires, body doubles, family favorites and mindless, monstrous husks that fans are still learning about. For the first time ever, the majority of Marvel's craziest family has settled on the mutant island of Krakoa. Since the character M has joined the X-Men's attack teams on important missions as seen in House of X, there isn't a better time to re-explore the wild St. Croix family tree as the X-Men prepare for a new future on Krakoa with some potentially dangerous members in their backyard.
EMPLATE
The villainous Emplate appeared in Scott Lobdell and Chris Bachalo's Generation X #1, when he attempted to drain the mutant life aura of the new member Chamber, which brought him to the attention of the rest of the team. It was later discovered that Emplate was actually Marius St. Croix, the eldest sibling of the family, who drained the life of his mother when his mutant abilities first activated.
Emplate has feeder mouths in his palms that allow him to drain a mutant's life energy, but he is also able to shift himself in-between dimensions to walk amongst the world unseen. He can also turn his victims into psionically controlled minions if he doesn't drain the entirety of their lifeforce. Emplate can become lost in the torturous dimensions he shifts to if he hasn't fed on enough mutant energy, which has resulted in his nightmarish appearance and dependence on the respirator he wears. Emplate also learned other-dimensional magics and abilities to further increase his powers.
M-TWINS
When Generation X was first formed out of the "Phalanx Covenant" storyline, Monet St. Croix/M was one of the first members introduced in Scott Lobdell and Joe Madureira's Uncanny X-Men #316, though she kept an emotional wall up around her and reacted harshly if telepaths attempted to enter her mind. When Empate first arrived, M knew more about him than she let on and she also seemed to share a connection with the mutant teleporter Gateway, who dropped off the mysterious mutant known as Penance on Generation X's front lawn.
M would spend a great deal of time with the team, however after a few years it was revealed that this M wasn't actually Monet St. Croix, but her younger twin siblings Claudette and Nicole. Since Monet was their father's favorite child, the twins decided they would live their life as Monet when she disappeared and went on to join Generation X. The twins were even able to merge with their older brother to create the evil being known as M-Plate, though they were eventually separated and returned to their own forms. However, the big question remained, where was the real Monet St. Croix?
MONET
Monet grew up as the favorite child of her father which caused some issues amongst her siblings after their mother died and older brother disappeared. However, when Marius first returned in his Emplate form, he asked his sister to join him. In true Monet fashion, she refused his offer with attitude, and Emplate viciously retaliated by transforming her using other-dimensional energies into a new body with red razor-sharp skin and claws that became known as Penance.
Emplate had kept Penance imprisoned for years so he could feed on her mutant energies, but Gateway rescued her and entrusted the mysterious silent mutant to Generation X. As Penance, she worked alongside Generation X for a while, though she continued to deal with the trauma of being imprisoned and fed on by her brother. While the M-Twins were able to banish Emplate after he transformed their older sister, they were initially unaware that Monet had been transformed into Penance. It wasn't until Larry Hama and Terry Dodson's Generation X#40 that the real Monet St. Croix would make her first actual appearance in the series after she was freed from the Penance form by her twin sisters, though they were forced to take her place in the transformed body.
PENANCE
Months later when Emplate returned to once again attack Generation X, an explosion at the Massachusetts Academy knocked out the weakened Emplate and somehow allowed the M-Twins to leave the Penance body and return to their individual forms, with the Penance body left as a seemingly empty husk.
However, the body was still able to operate on its own, and after leaving Generation X with the M-Twins, she was kidnapped and used to create the MGH (Mutant Growth Hormone) drug. She was later was freed by Ricochet of the L.A.-based superhero support group known as the Loners and remained with the team as Hollow. She would join the Loners when they moved to Avengers Academy, with the ongoing mystery of who now inhabits the husk of Hollow left unresolved.
KRAKOA
Both Monet and the M-Twins have shown that they are able to still transform into a variation of the Penance form, though it's unclear if Hollow is on Krakoa as a separate being or if the sibling's new transformative ability is a result of further merging amongst the St. Croix family.
Emplate was also welcomed onto the island of Krakoa, though his need for mutant energies is carefully monitored as he helps do the same for Krakoa, which also psionically feeds on mutants. The St. Croix family may have entered a new age on Krakoa in the Dawn of X era, but there might be a lot more to come from the X-Men's wildest family tree.