Entrance to catacombs is located on the square of fire and settlement of klesva. To enter one needs a Torch, which can be bought at the mercenaries shop for 10 s starting from level 5. Once a torch is lit you have access to the catacombs for 3 hours. In the first location you will find the bone dragon and a bunch of nightmare umertniks. Later about them.

Transfer time in catacombs is 1.5 minutes. You can make it faster by riding your steed, drinking elixirs of speed, etc. Catacombs have 3 locations: Entrance, Dark Corridor, and Forgotten library. Entrance has the nightmare umertniks, bone dragon, and the stash. Corridor has nightmare umertniks, library also has umertniks, though much less than the entrance or the corridor and guarantees you finding a pile of papers once every 24 hours.

Now, let's talk about bots. Nightmare umertniks cast Black Rage on you (converts 100% of their damage to hp.) can be removed if you cast Black Eclipse on them. After that they just become a strong bot with a lot of hp/crits/dodges, but nothing more than that. Don't forget that chances are that more than one umertnik will join your fight, so either hit very quickly, or have a few scrolls in your slots. On average they deal level 4-5 injuries for 14-17 hours.

bone dragon is a bit trickier. First of all, it casts Fear (turns your damage to 0), secondly - the phosphoric sickness. If not healed promptly you will infect others in your team and everyone will start losing 25 life points every 3 seconds. Both conditions can be healed with the scrolls of charm dispersal

Attention! if you die while the dragon is still alive - you will be resurrected on the other side as an umertnik and end up fighting your own team.

If you managed to kill the dragon (46800 HP) - you will get the stash key, Bone dragon ashes (if you have 20 SK and learned the manual), later several items will be added to the loot: Ring of Great Fortune, Ring of Irrepressible Anger, and Ring of Intense Reflection.

Inside the stash you will  find one of the following:
Goblet of Baddukh, Klesh Sarcophagus, Budge of Lost Soul, and Amulet of call - Stiagorn.