To view the concealed text, hold the left mouse button down and drag the cursor across.

Find and kill the prime evil that dwells within the castle

Go to the middle floor stair area.

Head North and go through the door on the right, then head East through one door, then South through the next one. You should now be in a room containing four tombs, with doors North and South.  Frob the NE tomb - it swings away to reveal a hidden entrance. Crawl through and pick open the metal door ahead. The Prime Evil is pacing back and forth at the far end of the room.

Collect all the Fire crystals you can find before you take him on -  there is a hidden stache up the ladder. The only weapons that hurt the Prime Evil are Fire arrows and Broadheads and you will need plenty!   Save game, then keep shooting him with Fire arrows until he disappears in a puff of smoke. If you run out of Fire arrows, then carry on with Broadheads until you finish him off. Objective complete!