There's a rusty crate in this room that's different from the rest. (For those of you who can't find it, it's at the northeast corner of the room.) Step on it, and it will lower. There's a switch at the bottom: pull it. Now a wall has opened in the room. There's an elevator behind the wall. Pull the switch there to lower the elevator. Be careful here, though. Quite a few Brown Serpents are waiting for you at the bottom.
There's an opening at the north side of the rocky area at the bottom. Go through the opening and turn left. There are stairs here that take you up on the ledge where a Brown Serpent was shooting at you earlier on. Go right on the ledge. There's a switch in the corner. Pull this switch, and the portal to the Tomb of Traductus opens downstairs. Go back down the stairs and enter the Tomb of Traductus.
Go down the long passage. There's a secret passage on the right at the first pair of stained glass walls, which leads to a dark room with some Dark Bishops and quartz flasks. At the second pair of stained glass walls, the stained glass on the left is actually a one-way wall. You can walk through it into another secret room with some health potions. Usually there's a Dark Bishop or two hiding behind the one-way wall, shooting at you.
If you walk to the end of the passage, the floor will lower. Usually I run there and quickly run back before the floor lowers, because there are many Dark Bishops down there!
After killing the Dark Bishops, jump down to the bottom. This is a triangular area; there's a door on the north side of the triangle. Enter the door there. There are three switches behind the door. The middle switch opens the area inside the triangle, where Traductus is. The other two switches open two other areas on the east and west sides of the triangle. It's not a good idea to pull all three switches, because there are Dark Bishops in the other two areas, and they might interfere when you're fighting Traductus.
The area on the west is a spiral staircase with a Falcon Shield at the top. But there's a trap there: when you get the Falcon Shield rooms full of Dark Bishops will open beside the stairs.
The area on the east also has stairs, that lead to a few small rooms. Walking inside the first room will open a few other rooms. You can find the Icon of the Defender here, among other things.
That's about it. The elevator on the south end of the triangle is activated now, so go back up and step into the portal that takes you back to the Necropolis. To get out of the Brown Serpents area in the Necropolis, pull the switch opposite the portal. This lowers the elevator where you came down on. Now you can continue your quest.