Welcome to the Ossuary. This is the first of two additional episodes in Heretic: Shadow of the Serpent Riders, the expanded version of the game. This episode is by far the most difficult to get a foothold in. Ammo supplies can be extremely tight unless things are done in the right order. Without foreknowledge of where items can be found, this level can take a few tries to successfully complete.
You begin in a small room, looking through a doorway into another larger room. Go to the next room and run through the sunken area, collecting the Ethereal Crossbow in the process. When you do, a crusher overhead activates. Take advantage of this crusher. You'll have many opportunities to trap enemies underneath it and can save a good deal of ammo by letting it kill them for you. There was a big crowd of Gargoyles on either side of you in the room where you started, that may have followed you out. Kill/crush them, then go back to the room where you started and look in the two side areas, where there are items. Back in the crusher room, there are exits in each direction, one of which is the Yellow Door. In another direction there's a small room with the Green Key out of reach on a shelf. The fourth exit leads outside to a small overlook with a railing that's too high to get over. Go back inside and look along the outer walls in the crusher room for a switch. Hit this, and the railing outside will be lowered.
Go outside and jump down to the ground. Out here is a tall structure in the middle of the area, a river that flows through two tunnels too low to get through that look into a dark flooded cave, and a small side room in one corner near the overlook. Ignore the Maulotaur roaming around; you won't have the ammo to fight him. Instead, go around back, find and collect the Yellow Key on a series of stone steps. Go to the side room near the overlook and take the teleporter in here. This goes to a tall room with a column. Activate any of the sections of wall with the blue and yellow tapestries, and doors will open. You'll see that this room is inside the tall structure, and the opened doors lead outside to a high ledge surrounding it. Dodge the Ophidians and step out onto the ledge. When you do, the column inside opens up, uncovering a teleporter that goes to the overlook outside. Instead of fighting the Ophidians, get them to run into the teleporter. After you've sent some or all of them to the overlook, go out to the ledge and collect the items if you didn't already, then take the teleporter yourself. The Ophidians you sent here should still be wandering around. Keep luring them under the crusher and, in time, you should be able to clear them with little use of your weapons or none at all.
In the crusher room, open the Yellow Door. There's a big crowd of enemies in here: get their attention and lead them under the crusher to kill or injure them. In the room inside the Yellow Door, there are stairways on either side leading up to ledges where there will be doorways open later. Continue down and ahead and open the wooden door. In here is a small room with the Green Door and a switch. Hit the switch and go back to the ledges. At the top of each, a doorway has opened. Avoid the one on the right, that has the dropoff inside, for now: it's a bonus area with several items, but also a very tough fight you won't have good weaponry for at present. Save it for later. Instead, go to the doorway on the left ledge to a small room, find and hit the switch inside. A low passage behind you will open, with a trail of items. Follow this, collecting the items, then go to the left ledge again. Now a nook with a switch inside has opened next to the doorway. Hit the switch, and go to the right-hand ledge. A teleporter here has opened. This goes to the dark flooded cave you saw outside earlier. Dodge all of the Ophidians and go to the far end of the cave. Approach the dark column you find: this is a lift that lowers, and on top is a Dragon Claw. Now that you have this, things will be a fair bit easier, and you can choose to take care of the Ophidians in here or ride the lift up right away, if you can tolerate being fired at while you wait for the lift. However you choose to do it, ride the lift up and follow the passage. You'll drop down onto the ledge with the Green Key.
Sometime, go back to the flooded cave, and carefully follow the wall between the lift and the nearest passage leading outside until you come to a section where the texture is different, between two hanging skulls. This is a hidden door to a teleporter that goes to a high ledge outside with items. Also, don't forget the items in the tunnels leading to the river outside.
Go to and open the Green Door. From this point on, your firepower shortage will mostly be over. In this small hall is a Hellstaff, plus you'll have collected a good deal of Runes so far. You'll see six recesses in this hall, three on each side. Activate the panel in the center recess that was on your right when you came in here, and step inside. When you do, the other five recesses will open, with small rooms and enemies inside. One of these will have a Map Scroll inside. Hit the switch inside the first recess you opened: this opens the stone door at the far end of the hall (watch out for the Iron Lich waiting on the other side), plus two teleporters in the other recesses. The teleporter opposite the recess with the switch just goes to the overlook outside. The one with the Ring of Invincibility goes to overlooks in the big room past the stone door you just opened. It's a bonus area you can go to right away, or later: it goes to the first of four overlooks, all of which are daisy-chained by teleporters that eventually lead back to the recess across the hall. Have weapons ready: there are Ophidians inside each overlook, but dealing with them now may make things easier later.
Go through the stone door you opened. This is a large room with a red column surrounded by a pit, and the aforementioned overlooks. Collect the ammo in here, then stand on the railing surrounding the pit. When you do, the floor of the pit will slowly rise to the level of the railing, releasing a few Iron Liches. (Tip: you probably have at least two Tomes of Power by now if you haven't used them yet. Using one of these to power up Ethereal Arrows, plus activating the Ring of Invincibility, may work better than Runes or Claw Orbs in dealing with the Iron Liches. The only headache is those obnoxious tornadoes they set off, but you'll be having to deal with those no matter what.) Activating the pit floor also opened one of those three big decorated panels. When you're done with the Iron Liches, hit the switch inside the opened panel. This opens a second panel; the switch inside there opens the third panel. The switch in there opens the red column, inside which is the exit portal.
The bonus area you passed over
Remember the doorway on the right-hand ledge in the room past the Yellow Door you skipped earlier? Now you'll have much better weaponry for it. Go to that doorway now, and jump down into the room inside here. Hit the switch inside the alcove: a section of stairway will rise into place. Hit the switch at the top of this, and follow the stairway up, hitting the switches and raising the stairs as you come to them. This eventually leads to a landing and doorway high on the wall opposite the room entrance. Note that when you step onto this landing, a lift in front of the room entrance will momentarily lower. That lift allows you to leave the room, if you need it.
Pick up the Gauntlets of the Necromancer inside the doorway at the landing. When you do, a small room behind them will open, with an ambush. Be careful as you're fighting the Gargoyles here: a door down in the first room has opened and released several Disciples of D'Sparil that can sneak up on you. Take care of all the enemies, go to the compartment that had the Disciples of D'Sparil and activate the back wall. This wall, plus the one on your right, will open, revealing a passage and releasing another Disciple-of-D'Sparil horde. Deal with them, and go down the passage, which leads back to the room where you began the level. Two more compartments in here will open, one on each side, with a few enemies.
At this point, you should have seen and cleared out everything except the Maulotaur outside. When you're ready to leave, return to the big room where the Iron Liches were and step into the exit portal.