Astel is one ... you will need to beat him in order to unlock the Age of Stars ending in the RPG. While not a particularly hard boss to take down the Naturalborn of the Void can be quite annoying ...

Astel, Naturalborn of the Void, is one the hardest hitting bosses in Elden Ring, thanks to its pesky teleports and almost predatory grabs. Astel is an optional boss, but it provides you with a ...

then explore the zone to find a coffin that will take you to a zone with a bossfight, this time against Astel, Naturalborn of the Void.