so jacen’s sequel trilogy verse goes like this:
he is the eldest son of han and leia, younger twin brother to jaina, older brother of ben and anakin solo, older cousin to ben skywalker. he, his sister, his brothers, and his cousin all went to train with luke at the jedi academy as soon as they were old enough. jacen, like in the eu, loved learning and was proving to be exceptionally powerful and knowledgeable about the force, standing out brightly among the other students. but as jacen’s power grows, so does the danger he presents; lumiya, dark lady of the sith and luke skywalker’s mortal enemy who is looking for a way to best him once and for all, begins to take an interest in the eldest solo boy, sensing his immense power in the force, while at the same time knowing that snoke, a dark side force user, has been trying to coax out the darkness from his younger brother, ben, another powerful albeit less developed force user, seeing as ben had a natural darkness that was easier for snoke to manipulate and groom. she sees this as her opening to taking jacen for herself and make luke skywalker pay by turning his nephew to the dark side, waiting in the shadows for her moment to advance, all the while watching him as he becomes a powerful jedi.
he and jaina become jedi knights, but mostly stay at the academy, continuing to help luke train the learning padawans, jacen becoming his cousin, ben’s, master. and it’s then that snoke’s plan to drag ben solo into the darkness takes full effect; lumiya, knowing what ben solo would do if he were threatened, plants the dark visions luke has about ben solo, which ultimately leads to the destruction of luke's jedi temple in the dead of the night. jacen, having been asleep when it was happening, is crushed under the rubble, and is assumed dead. when snoke’s first order arrives to ensure all the jedi are dead and accounted for, vergere, lumiya’s right hand who’d been working with the first order on her orders, senses jacen’s (still alive) presence beneath the rubble and rescues him — only to be brought to lumiya herself, who has other plans for him.
once brought back to full health, jacen, who is now in lumiya’s captivity and believes his brothers, sister, and padawan are dead, is forced through round after round of endless torture in an attempt to break his soul. jacen, at first, fights against it valiantly; but there’s only so much someone can take before they reach a point of no return. in his time being tortured, both lumiya and vergere groom and coach jacen through the pain, making him use the dark side to accept and harness the pain (physical and otherwise) to filter raw force energy through himself; this is a cycle that is repeated over and over again until jacen begins to feel fortunate for their presences, having been made to feel like they were helping him despite them being the ones putting him through torture in their captivity. he willingly becomes lumiya’s sith apprentice, and fully accepts her teachings until he rises as the new dark lord of the sith, darth caedus.
at the same time, ben solo has become kylo ren, a formidable dark side force user, who darth caedus doesn’t know is still alive due his isolation in captivity. however, caedus, now ready to force the ways of the sith on the galaxy, senses this rising dark power and is determined to snuff it out — or force it to join him in his vision for order of the galaxy.
