This level occupies the map slot MAP29. For other maps which occupy this slot, see Category:MAP29.
From the blue skull key go south, but instead of descending the stairs look at the edge east of the stairs: Walk over the edge's southwestern half to the southeast to land on a ledge (sector 139) with a computer area map on it.
From the southwestern entrance (north of the red skull key) of the great cave in the northwest of the map, descend some stairs. At the first opportunity, drop down on the floor to the north. On the nearby (southernmost) metal cube, push the satyr face on the southern side. Drop down into the blood river to the north to see that the wall behind the health bonuses on the river's eastern side has opened, revealing a nearly circular room (sector 613), containing a box of rockets, an energy cell pack, four medikits, an arch-vile and a light amplification visor. In addition there are two sergeants (on medium skill levels) or two revenants (on hard skill) who spot and attack you when approaching the room.
From secret #2, follow the blood river to the north and use the teleporter to get back to the cave entrance. Descend all the stairs and push the satyr face on the southern side of the nearby (westernmost) metal cube to lower the lift in the north. Ride it up, jump over the blood river and proceed to the easternmost metal cube with a lit satyr face (pushing it opens the metal bars in the southeast) on the northern side. Drop down the edge to the southwest to land on a ledge (sector 251) with a berserk pack and (skill level dependent) one, two or three chaingunners.
After opening the metal bars mentioned in secret #3 and leaving the great cave using that exit, follow the path to east. After ascending some stairs and entering a door, (you need to push the lit satyr face to the north right after this door in order to access the yellow skull key later) go south and drop into the blood lake. Go east and take the southern path at the fork, descending some blood fall building stairs. At the end of the path you see a soul sphere. Approach it to register the secret (sector 216) and let a monster (a revenant on easy or a mancubus on medium and hard skill) teleport in behind you.
From secret #4, climb up the blood fall and take the northern path to the east. Go north at the first opportunity between the vine covered walls and under the demon faced flagstone with a pentagram on it. Follow the bend to the west, ascending some stairs and enter the door at the end of the path. Descend the stairs behind the door. At the blood pool's northern end is a satyr face switch, approach it walking to the northwest, thus hugging the western blood fall. At some part (there is a gap in the automap) you will walk through the blood fall, entering a secret passage (sector 599) with a skull switch at its end.
Push the skull switch in secret #5 and the satyr face switch at the blood pool's northern end. Go south and ascend the western stairs. In the room south of the door (opened by pushing the satyr face), a niche (sector 586) in the western wall behind the super shotgun has opened (by using the skull switch). There you find a BFG9000.
After opening the third door in front of the map exit room, where the player first has to use the blue, then the red and then the yellow skull key, two teleporter rooms open up in the wall surrounding the area the player is in: one in the center of the northern wall and another one in the center of the eastern wall. Some monsters teleport in those rooms and start attacking the player.
As soon as the player approaches one of the four medikits or the green armor in front of him (behind the yellow door), he triggers (Linedef 4061, Action: 19 - W1 Floor Lower to Highest Floor, Tag 35) a wall to rise in the south of that area, thus blocking the only retreating opportunity. Entering the eastern teleporter room and stepping on the teleporters triggers (Linedef 4059, Action: 38 - W1 Floor Lower to Lowest Floor, Tag 35) that wall to lower again.
The player does not need to cross any of those lines before he fights the monsters teleporting in, but both lines have to be triggered before exiting the map (linedef 4061 to approach the map exit and linedef 4059 to use the switch east of the teleporters to remove some bars blocking the map exit).
As the lines can triggered only once and it is possible to trigger the already lowered wall first to lower and then to rise, the player can lock himself in that area, unable to return to the rest of the map to collect remaining items.