How to start Leviathans DLC ?!?

New player here! I found a huge fleet of leviathans in a section of my empire that i haven’t claimed yet, but it’s a huge fleet of leviathans there.

I’m only 50 years in, i kinda know this is supposed to be some endgame content, but how do i start the event?

As it seems they are all friendly and they don’t attack or anything. Only a single fleet of theirs is surveying my systems i guess.

How do i proceed from here?