The Mikaelsons made the ultimate sacrifice on Season 4 finale of The Originals, breaking their familial bond once and for all in an effort to save Hope from the grip of the Hollow. By allying themselves with Vincent and Marcel in "The Feast of All Sinners," the Mikaelsons were able to save Hope on The Originals, but it turns out that defeating the Hollow means the end of the Mikaelson family motto "Always and Forever."
With the Hollow gaining more and more power every passing moment, the window to save Hope from the Hollow's possession is rapidly decreasing. Luckily, Vincent has a plan: he'll perform a magical ritual to get the Hollow out of Hope, but to keep the Hollow from re-possessing Hope he's going to need to put the Hollow into four immortal beings related to Hope, aka the last four Mikaelson vampires. Sounds great, right? Not so fast, there's a major catch. In order to keep the Hollow from regaining her power, once the Hollow is separated into Elijah, Klaus, Rebekah, and Kol, they can never see each other again. To save Hope on The Originals, the Mikaelson family must be separated once and for all. Even worse: they can never see Hope again, which means she'll grow up missing her father, aunt, and two uncles, while Hayley and Elijah's relationship ends.
After the ritual was complete, the Mikaelsons followed through on their promise. They each went their separate ways, with only Freya left as a possible go-between. Hayley went to Mystic Falls where Hope enrolled in Karoline and Klaus' school for the supernaturally gifted. Kol escaped to San Francisco with Davina, and Rebekah fled to New York City where she was joined by Marcel. Sounds pretty happily ever after, right? Wrong. To ensure that he does not run to Klaus' aid when the original hybrid relapses into his horrible, murderous ways, Elijah had Vincent and Marcel erase his memories of family and loyalty and love. He has no memory of his life as a Mikaelson, leaving Klaus truly alone.
