The original full-length version of Heretic only had three episodes. Being the original finale of the game, this level culminates in a battle with D'Sparil, the game's only true boss enemy. A later expanded version of the game included two additional episodes and was released under the name Heretic: Shadow of the Serpent Riders.
You begin in a large square water-covered room, on a platform surrounded by a watery pit. Jump down into the pit and collect the items, one of which is the Wings of Wrath, then use the Wings to fly up and out. In the four corners of the room are teleporters, three of which will be covered up. Before continuing, there's a secret area nearby: follow the wall until you find a section that's not flowing to the left and has a grating at the bottom. This is a hidden door to a room with several items.
Series of rooms
Step into the teleporter, clear out the enemies in the room to go to, and step into the teleporter in here. You'll be back in the main room, except now the pit has filled up, and a second teleporter has been uncovered. Step into this teleporter and clear out this room like you did the last one, and enter the teleporter in here. You'll be in the main room again, and a third teleporter will be uncovered. Step into this one, and hit the two switches in the room you get sent to. These open two doors in series leading to an adjacent room with a teleporter. Enter this one, and you'll be back in the main room, and the last teleporter, which leads out to the main arena, will be uncovered. This teleporter is the one next to the secret door; if you want to fill up on ammo in the rooms you went through, it's the other teleporters you use. When you're ready, enter the fourth teleporter. (Tip: whenever you teleport back to the main room, yuo'll be facing the next teleporter you need to take.)
You'll be in a huge outdoor area, on top of a structure in the shape of the trident symbol that you've seen throughout the game. Collect the items up here, then jump down and collect the Quartz Flasks in the pool surrounding the structure. Before joining battle, take a run around the area. In addition to a bunch of ammo, you'll see square pedestals that are dark on top. These are teleporter destinations that D'Sparil will be using later. At the far side of the area there's a building surrounded by a deck out of reach. Look around the right-hand side of the building and you'll find Wings of Wrath in the corner by the outer wall. You can use these, either right away or later, to fly up to the deck. If you do, go to the ends of the deck on both sides of the building and look for the blue and yellow tapestries. Both of these are hidden doors to small rooms, each with a teleporter. One goes to the structure where you first came out here; the other back to the room where you began the level. There's also a Ring of Invincibility in one room.
It's time for battle! Fighting D'Sparil will occur in two phases. At first he'll be mounted on a Chaos Serpent: a big green lizard-like creature that shoots fireballs. It'll take about as much firepower to destroy as a Maulotaur but isn't much trouble. When the Chaos Serpent is defeated, D'Sparil will be momentarily subdued but will quickly come around. Now the real battle begins. Dismounted, D'Sparil will now be doing battle with you directly. He fires a nasty energy orb, can summon his Disciples, and (contributing the most to the difficulty of the battle) frequently teleports away to the destinations throughout the area. Above all else, concentrate on D'Sparil. Don't spend too much time on the Disciples, because he'll just summon more. Continually run and strafe, and keep the horde of Disciples in view, and you'll be able to avoid most of their weapons-fire fairly easily. Keep your Quartz Flasks selected so you can quickly activate them. There are high-power items like Tomes of Power and Flame Orbs available, but you'll probably find it more efficient to use the Hellstaff and Dragon Claw in their normal non-powered-up modes, because of their accuracy and fast firing rate. Be extremely careful if you're using the Phoenix Rod: Disciples often get in the way and it's very easy to end up injured or killed by backsplash. Finally, don't forget the Wings of Wrath and hidden areas mentioned above if you need them.
If you keep your attention on D'Sparil and re-acquire him when he teleports away, you'll defeat him within two or three minutes. He'll be overwhelmed by the energy field he was using, and then promptly disintegrate, his bones clattering to the ground. After he's defeated, look along the outer wall behind the trident structure, and you'll find a passage to a small area that wasn't there before. Hit the switch inside here: a wide stairway up to the deck and building will be raised. When D'Sparil succumbed, a large door into the building opened. Inside is a nice, large, colorful exit portal. You have now completed the episode. If you're playing the original version of Heretic, you've also completed the game altogether if you played the episodes in order.