Temple of the Ages

You can find the Temple of the Ages in The Black Curtain area. From the Temple of the Ages you can gain access to the Underworld and The Fissure of Woe by kneeling in front of one of the god statues (Grenth or Balthazar), which will summon the god's avatar. Pay 1 plat to the gods' avatar and they will take you to the UW/FoW.