Welcome to the fifth and final episode of Heretic: Shadow of the Serpent Riders. This level will be a good deal easier to get going in than the prior episode's first level was, although it has very tough combat in a few areas. You begin in a large, deep outdoor area featuring a river, a series of progressively-taller steps and ledges leading up along the far wall, and the entrances to several buildings, one of which is the Yellow Door. At present, most of the series of steps is out of reach because of steps that are too high. There's a lot of ammo available outside, giving you a good start on building up your stores.
After clearing out the enemies (you can leave the Golems on the steps for later if you want), enter the building you can get to. In here is a dimly-lit good-size room with a bunch of irregular-shaped columns, and overlooks to the left and right. Watch out for all the Golems clogging the overlooking areas: they're the flashie-flashie variety that launch the flaming skulls. Go up to the platform at the far end, where you'll find an Ethereal Crossbow. Turn around right away when you pick it up: when you stepped onto the first step, half of the columns lowered, releasing Undead Warriors.
An interesting quirk in the mechanics of the level occurs here. Normally, when you pick up the Ethereal Crossbow, the remaining columns will raise up and reveal compartments underneath them. And then, when you step onto any of the spaces where the first columns and enemies were (there are Time Bombs of the Ancients and other items there), the floors of the compartments will rise up to the level of the room, releasing additional Undead Warriors. But if you step onto the spaces where the first columns were before picking up the Ethereal Crossbow, the remaining columns won't open when you pick it up.
Whenever you step onto the platform where the Ethereal Crossbow was, four lifts in the corners of the room will momentarily lower, which lead up to a series of corridors connecting the overlooks. Ride a lift up, and clear out all the enemies from end to end. At the midpoint of the corridors you'll find the Yellow Key. You'll see that either end of the series goes to those openings way up on the wall outside.
Go back outside. When you stepped up to the Ethereal Crossbow earlier, some of the steps lowered, allowing further access. However, most of the higher steps past the Yellow Door will still be out of reach. Enter the Yellow Door. In here is a big labyrinth of narrow passages at various levels. Thoroughly explore and clear out the area. You'll find the Green Door, a Dragon Claw, and two tall rooms: one more or less directly beyond the Yellow Door next to the Green Door, and the other nearby off to the right. When you step into the tall room near the Green Door, the lights will go out inside, plus several portions of the labyrinth will go dark. In the room that made the lights go out, find and hit the switch: this lowers a lift opposite the room. Step onto the lowered lift.
This is all you needed to do in here, but there are some extras available at a cost. When the lift rises, you can get the Morph Ovum. If you do, it triggers a filthy-dirty ambush of Iron Liches and Disciples of D'Sparil here in this room. It's easiest to leave right away and then fight from the doorways. Be sure you have the Dragon Claw. Whenever the Iron Liches fire off those obnoxious tornadoes they're so fond off, leave until they fizzle out or go around to the other doorway and fight from there, otherwise you'll get thrown around too much and risk being killed. You'll find various items in the compartments the enemies were in.
Go back outside. When you stepped onto the lift you lowered back inside, it lowered some of the steps leading up, allowing access to most of the remaining area. You'll find one more doorway, plus a few more steps out of reach leading to an alcove with the Green Key. Enter the doorway and then activate the wall directly ahead: it's a hidden door to an area with items. You need to find and enter this area in order to complete the level. (In most cases it's poor level design to make secret areas compulsory, but in this instance it's possible to obtain a Map Scroll before you need to get into the secret area, so at least you'll know it exists. Plus, it's the only one in the level.)
To the right is a shallow lava-filled chamber with overlooking areas on either side. Be very careful of the fireballs being continually spit up in here. At the far end is a small room filled with hanging skulls, a Hellstaff, and the Blue Door to the level exit. When you stepped into the secret area you just opened, a walkway crossing over rose into place. (This walkway is necessary to reach the Blue Door. That's why you needed to get into the secret area.) This room can be a bit tricky. You'll notice the overlooking areas are on the same level as the walkway. You'll need to get to the overlooks: the easiest way is to run over to them from near either end, where they're closest to the walkway. Also, at the midpoint of each overlook is a lift that lowers if you fall or step onto the lava. Before jumping over to the overlooks, clear out the Undead Warriors. Be careful not to get shot in the back or rained on by fireballs. One trick is simply to leave the room and go outside. You will have seen teleporters at the near end of the overlooks; the enemies will wander into them and be sent to just inside the doorway, where you can fight them as they arrive.
Ambush and Green Key
Back in the lava chamber, jump over to one of the overlooks, then run right away to the teleporter at the end. Entering either overlook causes areas in both overlooks to open, with a bad Iron Lich ambush. Like with the Undead warriors earlier, they'll wander into the teleporters and be sent to the doorway, where you can avoid their attacks more easily. When you teleport out, go outside: when you first entered the lava chamber, it lowered a step allowing access to the alcove with the Green Key. Collect it, then jump down to the near end of the river. When you enter the Green Key alcove, a lift disguised as water lowers, leading to a flooded cavern downriver. (You probably saw enemies wandering around in here through the gap under the wall.) There's a Phoenix Rod in here.
After taking care of the Iron Liches that teleported from the overlooks, go back to the overlooks. There are many items in here, including Inferno Orbs and the Map Scroll that would have allowed you to find the secret area that raises the walkway. (If you hadn't found the secret area, you would have had to jump down to the lava and taken the lift to the overlook.) In one overlook is a switch: this uncovers the exit portal in the exit room.
Now that you have the Green Key, go back to the labyrinth and open the Green Door. In here is a Firemace, completing your arsenal. Hit the switch inside and go to the second tall room (not the one by the Green Door, the other one). The switch you hit opened an area with the Blue Key and a Mystic Urn you're about to need. The entrances to the room close off, and two more areas in the room open, with an especially nasty Disciple-of-D'Sparil ambush from two directions and little room to fight. One thing that works well is to activate a Tome of Power (you should have a couple by now) and use the Firemace you just received. When powered up, the mace projectiles are super-sized, are lobbed out slowly, have poor aim, but cause instant death to most enemies. It's almost impossible to miss in this cramped room. The exit to the room is a hidden door in one of the areas where the Disciples of D'Sparil were; rub against the walls and you'll run into it.
Exit and extras
Return to and open the Blue Door. (If you don't see an exit portal, find and hit the switch in one of the overlooks across the lava nearby.) Inside here are Wings of Wrath. Before leaving, go back outside and fly up to those two high ledges across the river. There are items on each one including a Ring of Invincibility. Compartments behind them will open, releasing a few enemies.