Garamon is a npc north of the entrance of Underworld. You can speak to him. Then he speaks words and you get a quest. The quest when done lets you go deeper into the dungeon without being on the Sacred Quest. The Sacred quest is easier though.

Word Garamon's answer
Hello Have a nice day!
Secret He who pays close attention to the walls may notice something unusual.
Teleporter You will find many within the dungeon. They will facilitate your travels.
Vines Aaah yes! Tricky things they are. Try to find something that could burn through them.
Burn I can tell you right away it's not fire based. Surely something within the dungeon will yield what you need.
Abyss Its entrance is protected by stone guardians who will only grant passage to the carrier of a Tripartite Key.
Stone guardian They will not let you enter the Abyss unless you can present a Tripartite Key
Key Its three parts you must find and reunite as one
Parts Two can be found hidden in secret rooms within the Underworld. The third you must take from a shadow of evil.
Shadow of evil A most foul traitor. Once you have the first two parts, challenge him for the third! He dwells beyond the void in the Shrine.
Shrine Find your way there through the dungeon. You must use a teleporter to reach it.