The second mission in Assassin's Creed: Odyssey Fate of Atlantis is called the Guardian of the Gates.
Once you defeat Cerberos, you will be taken to its owner. After a long conversation, he will leave you with his allies. He isn't happy with the fact that you have killed his dog. In addition, he will not believe in your strength and that you did it yourself.
Before disappearing, Hades will leave you with a few enemies. To defeat them, you must first enter the place indicated in the picture above. This is the only way to make them vulnerable. If you don't do that, you won't be able to deal them any damage.