where is fissure

is Underworld and fissure the same thing? if not, where is fissure? is it inside underworld? or wut?

they are two diffrent places, you enter both from the Temple, but you have to pray to diffrent statues in order to get into them, I believe Belthazar is the one for FoW, and cant remember underworld.

Grenth is Underworld, Balthazar is Fissure of Woe. They are just two different places for end-game PvE. It's nice to take a trip fo FoW for a break from the Underworld, and vice versa. At least I assume that's the point of putting in two different zones.

Also the special armor available in both zones have different appearances, I think.