The run down of the modern Layla story goes back to Origins. She worked for Abstergo and went "rogue" after discovering the memories of Bayek and Aya, abstergo pretty much sent a hit squad after her. William Miles, the father of a previous protagonist and leader of the Assassins, gets to her first and convinces her to join the assassins. In Odyessey we see her trying to track down a POE. She has her own team and without going too much into spoiler terrority, it turns out that someone from the pass is a holder of that POE(Piece of Eden) and gives it to layla. In the DLC Fate of Atlantis we see the Isu putting Layla through a series of tests to see if she was the one that was meant to hold the POE. However, the POE ends up changing and she soon becomes violent.

After everything is said and done, William pulls her from the field and asks Shawn and Rebbeca to keep an eye on her as they had previous experience with the Apple of Eden from their time with Desmond. One of the reasons Layla looks like a piece of crap is because of the affects that specific POE is having on her.
To clarify some more, and I'll be delving into spoilers for AC 2-3. In 2012 the world was suppose to end by a solar flare, but the isu built devices to stop it as that is how their civilization ended. It ends up turning out the only way to protect the world from the flare was to release an isu that wanted to enslave humanity, from the POE. Desmond did and it ended up killing him.

Back to Layla, another solar flare is threatening to destroy the world again, so Layla, Shawn, Rebbeca, and Williams are looking for another temple they can use to stop it from happening.

Trying to understand it all can be a bit difficult as some things are only mentioned in other media such as books and comics.