I've never been able to find where does the key to "secret jail door" fits so can anyone help me with that?

The second problem is a bit more complicated
My level 21 barbarian(solo) is having trouble with the mists in the 1st lvl of watchers keep and in the temple(cant remember the name but where the witch who raised Yaga Shura lived) how do i deal with the problem, as a side note, i have no trouble with level drains thanks to the barbarian rage, the problem lies in the excessive damage dealt by them along with their vanishing acts!

What is your AC? Did you get the Crimson Chain or the Shadow Dragon Scales? Either will decrease your AC significantly.

The key is used in Saradush, in order to meet Gromnir. As there are several ways to meet him, you may end up not using it at all.

Nope Dundee, no shadow dragon scales, i've not imported my char, i made a new one. What and where is this crimson chain. I've got 2 stars in flail, bastard swords, 1 star in smictars and sword and shield, and 3 stars in 2 weapon style. I've got flail of the ages and belm ! poor me !

The Crimson Chain is sold by Ribald in the Adventurers' Mart in Chapter 6-7 as part of his special stash, so no chance of getting it now.

You have defeated Gromnir and his men right? I guess you are using the Grandmaster's Armour now. Hmmmm... there is not much you can do but take it in your stride I guess. The Vampiric Mists are not that particularly tough. Just gulp healing potions while you wait for them to appear.