To begin this quest simply meet your mother at the symposium. As you approach her a cut scene will initiate in which Aspasia is warning your mother of cultist assassins on the island. Navigate the dialogue options and agree to seek out the assassins. They are likely hiding in a cave system in the south of the island. Make your way there where it is marked on the map. As you approach the specified area use Ikaros to scout the area and identify the cave entrance. Once inside, make your way to the depths of the cave. Along the way you will encounter a few Athenian guards which are not difficult to dispose of. Finally, when you reach the furthest point in the cave you will find the cultist assassins at an altar. One of them is swift in combat and the other is more of a tank. Devastate the smaller assassin with bow shots before you move down to focus on the tank. When they are both dead, loot the corpses to find evidence and return to Myrrine.