Season 1 of Legacies ended with the last twins of the Gemini Coven (Josie and Lizzie), finding out about The Merge. Despite how long Caroline (their mother) has been looking for a magical cure so that her children won't have to do it, and how much Alaric (their father) has hidden information about it from his daughters, the Merge seems inevitable. Jo (their briefly resurrected bio-mom) tells her daughters to fight for each other, to remember that she said that, and that they will understand it later.
Related: Legacies 10 Best Reveals of Season 1
Later is now. In the Gemini Coven, when twins turn 22, they have to do the Merge. The twins fight, and the stronger twin both kills and absorbs the weaker one, becoming more powerful. We know from The Vampire Diaries
that Lizzie and Josie have been extremely powerful from a very young age. We see some hints of this in Legacies,