This item has dropped twice for us off a named in the sewers called "a swirling green ooze", which was approx lvl 57/58 and ought to be an easy fight for any group capable of holding a camp in the sewers, since it wasn't much harder than an average "pool of slime".
It spawns in the first room to the left after the first T-intersection in the sewers, if you enter from the courtyard side rather than dropping in via the AoF room.
When camping the area, avoid the temptation to go too far in unless you have a very good team - a see invis mosquito spawns after the second room (i think) and complicates CR, which can otherwise be done easily with invis.
Aye ha' dream'd fer something like thes ta drop fer meh warrior bruddas en the waist slot. 28 AC/80hps en a waist item, plus a' tha o'her goodie stats? Looks like aye'll ha' ta help some warriors hunt thes en CT sa aye can ha' some mar' bonus ta meh CH =P. Har har har /em raises his Brell's Blessed Stout in the air to congradulate any warriors that can get their hands on it.