In the zone Tipt, in the reservoir area, there is an innocuous NPC named Minar. Minar has one quest as far as I know. If this quest had a Journal entry it would probably be called Entrance to Sewers of Nihilia, the Purifying Plant. Its a simple quest where you gather and give four "Sliver of Animated Ore" (item 120872) to him and he then simply tells you that you can enter the zone. However, its actually not necessary to do this as you can click on the zone in (the nearby pile of rubble) at any time and get ported in.
The zone itself is an undeveloped zone, i.e., THF has not populated it. Its a largish zone with very clear pools of water that must be swam in to get to certain parts of the zone. There are no mobs that I could see although our map shows the location of where would be Ansharu.
In Live EQ, Nihilia is a part of the Gates of Discord progression. The entry is acquired by doing a quest given by an NPC in Barindu. Since Barindu is (I think) not an active zone in THF, I assume Minar was created and placed to make up for it. Minar does not seem to exist in Live EQ.
None of the other three zones of Nihilia appear to be active at this time.