Character: I must be getting close!
Hell is the fourth and final set of levels in Diablo I, consisting of levels 13-16.
Shrines appear as Cauldrons that give random magical effects or other enchantments like the Goat Shrines did in the Caves. The walls in the area are made of bone with their interiors filled of pulsating blood, and there are no doors to block monsters out of a room.
Hell looks more depressing and horrific than the other levels. Corpses litter the areas, either impaled through stakes or hanging on walls. Packs of Hell's most fearsome demons and monsters rove the halls, supported by powerful mages. Staircases descending further into the carnage appear like huge halves of ribcages, except the entrance to level 16, which says "DOWN TO DIABLO" and is a circled pentagram with a hole in the center.
As with other level sets, the first level of Hell, level 13, has a staircase up to town. The entrance is a blood-red crevasse in the earth, behind Pepin's clinic. Mousing over it says "DOWN TO HELL".
Hell has two multiplayer quests, and four single-player quests. Three of the four quests will appear in a given singleplayer game.
The singleplayer-only Warlord of Blood quest can be started by reading the Steel Tome on level 13. The Warlord of Blood monster is located in the room containing the staircase to Level 14, some armor racks, and some weapon stands.
To access level 16, Lazarus must be defeated. In singleplayer games, his staff will be found somewhere on Level 15, at which point the Archbishop Lazarus quest can be started by speaking to Deckard Cain in town. This opens a red portal near the entrance to Level 16, leading to the Unholy Altar where Lazarus can be found and defeated to unlock the entrance to Level 16. In multiplayer games, Lazarus will simply spawn in a set-piece room somewhere on level 15.
Warlord of Blood and Lachdanan share a quest group, such that one or the other will appear in a given singleplayer game.