The Crypt of Kryl'Feijan (or "Dark Crypt" as it is known in the game) will appear in a random location on the western continent. It is a triggered encounter, similar to the Unknown Tunnels, rather than a visible zone. You will only have the option of entering once, while traveling on the overmap and being level 24 or higher.
The Dark Crypt has 5 levels, the first 4 of which are small, consisting of largish rooms connected by narrow corridors. Most of the area is unlit. These levels are very dangerous because of their size. All the enemies will be concentrated into a very small space, forcing you to fight many of them simultaneously. (Many people skip this zone when it's offered, because of the deadliness. If you wish to tackle this zone, it is recommended that you stockpile some blight resistance items and bring them with you when you're level 24+.)
The opposition consists mainly of Rhaloren (renegade elven) corruptors and blood mages and melee guards, with a smattering of minor demons. Most of the mages use various corruption spells and they love to teleport you into the middle of a room, where you will be pummeled with massive amounts of blight damage. Watch your encumbrance -- a weakness disease can keep you from moving by reducing your carrying capacity.
At the bottom you will encounter a group of corruptors with a time limit displayed on the UI. Killing each one increases the counter a little bit. If you fail to kill them within the time limit a very powerful demon will be summoned, and you will have to fight it. However, if you are successful in killing the corruptors you will free a human woman named Melinda who was to be sacrificed to summon the demon.
If the timer runs out Melinda dies and you will have to fight the demon Kryl-Feijan.
If you saved Melinda you will then have to escort her out of the crypt by keeping her safe until you find the stairs.
WARNING: if you are playing an alchemist or otherwise have a pet you will need to make sure that Melinda is directly adjacent to you when you attempt to use the stairs; otherwise you will be unable to leave. An easy way to keep her safe is to dig a spiral path in the bottom walls, then teleport out causing her AI to become stuck there in the spiral path. After you kill the enemies, come back for her and lead her to safety.
After escorting Melinda out of the dungeon you will be able to find her in Last Hope by visiting the rich merchant's house (in the far south). Her father will give you a reward (randomly chosen artifact), and you can start a romance sub-plot.