The Abyssal Trinity area allows you to turn in Vial of Soul Fragments (Blue, Green, Yellow) in exchange for killing 3 mini bosses. These 3 mini bosses drop different tokens respective to each boss; the shell of Tsovinar, the shell of Vashna, and the shell of Seth. These tokens can then be sacrificed to kill the final boss, Maiden of Death.
Your first task should be to collect Vial of Soul Fragments which can be looted from certain bosses around Cyntara. In order to kill one of the mini bosses, you will need 1 Vial of Soul Fragment (Blue), 1 Vial of Soul Fragment (Green), and 1 Vial of Soul Fragment (Yellow). You need to kill 3 mini bosses in order to access the final boss, so you will need to collect 3 of each color of Vial of Soul Fragments, but you can kill the mini bosses one at a time so you don't need to collect them all at once.
Once you have 1 of each of the Vial of Soul Fragment, head into the Abyssal Trinity teleport and sacrifice the soul fragments by putting them on the stone counter and pulling the lever. They must be sacrificed in a certain order, so use the picture below for a guide.
The west, north, and east levers are the mini bosses, and the south lever is for the final boss.