You begin in a small chamber. Step forward and the wall will open, revealing a large lava-filled cavern, with platforms here and there. You'll notice some of the platforms have a texture that's more brown/gray, and the others brown and orange. The brown/gray ones will sink into the lava when you step on them. You'll also meet another new enemy here: the Sabreclaw. These don't shoot anything, and are usually very little trouble unless you get cornered by a gang of them. They drop Runes.
Across the cavern you'll see two large doorways, plus there's a smaller door to the left. Go across to the doorways, and to a long room with columns, a long window high on the left-hand wall and a lava river. (Tip: when crossing the lava cavern to get here, go to the right: this will save the stepping stones to the smaller door, which you'll go to later, and you'll spend less time running in lava.) Look in front of the middle of the three lava falls, and collect the Yellow Key.
This was your reason for coming in here, but there are also a few bonus areas. Those two pointed platforms in front of the lava falls are lifts that activate when you step on them. Take the lifts up to the top of the structure: there's an item in the lava pool. After getting this, jump off the back of the structure: when you walk over the edge, a door to a secret area behind the structure will open. Next, follow the lava river down to the far end, where it drops down into a lower room. There are a bunch of good items in here. Use the Wings of Wrath you find in here to exit. These are the only ones in this level, so enjoy them while you have them. (Caution: don't go back down there a second time later. Without Wings of Wrath you'll be stuck, unless you have a Chaos Device.)
After you have the Yellow Key and are done in here, go back to the lava cavern and to the smaller door. Your Wings of Wrath should still be active, and you can fly over the lava and collect items. Go through either Yellow Door, and upstairs. The other stairway you see is from the other Yellow Door. Find and hit the switch here, and another stairway forms, going up to the door that was out of reach. Go upstairs to the door, and it will open, leading outside. Then look behind you: the panel with the switch is actually a hidden door that just opened. There are a bunch of enemies inside, but also several good items.
Outside, past the door, go left. You'll find a lava pool (there's a Ring of Invincibility on the other side) and a building with a second building inside. Your task now is to open the door to the inner building. First, go around to the back of the inner building (where the enemies were firing at you through windows) where there's a Map Scroll.
Go back to the door that initially led out here, and continue that other direction. You'll pass the green Door on your right. Continue this way: it goes down and off to the right, and ends in a room with two wooden doors and another passage leading down. Look at the rooms inside both wooden doors: one has an item, the other a switch. Hit the switch to open the inner building back outside.
Before going back, there are two bonus areas nearby. In the two-wood-door room, look for a section of wall that has a large gold decoration instead of the small ones like everywhere else. This is a hidden door to a secret room with a Hellstaff and other items. Go down that second hallway, which goes off to the right and leads to the room with the long window looking out into the lava-river room. Activate the wall with the blue banner to open a hidden area. There are three small rooms here, connected end-to-end, each with an item.
When you're done, go back outside to the outer+inner buildings. Collect the Green Key inside the now-open inner building. You'll also find your first Gauntlets of the Necromancer of the episode in here. Go open the Green Door. Take a look at that recessed area in the back-left corner in the room inside, then hit the switch in the back-right corner. The switch momentarily raises the pillar in the middle of the room, revealing the exit portal.
Before leaving, there are some more bonus areas you can now get to. When you hit the switch, a door in that recessed area in the other corner also opens momentarily. Go inside there; there are items. When you pick then up some of the pillars will open and you'll get an ambush. Activate the wooden wall at the back of the room. The teleporter inside goes to the structure in the lava-river room. Last, go outside to the lava pool near the outer+inner buildings, where the Ring of Invincibility was. A door here is now open. The passage here (watch out for the Iron Lich) leads down to the chamber where you began the level. There are several items along here. When you've gathered items and seen everything you want, go back to the Green-Door room, hit the switch and step into the exit portal to end the level and continue your mission.