I want to know so that I may use characters of Evil alignment in my reputation 20 party without all of the complaints. If you know please post.

As of record, no-one has ever found it in the game, sorry to disappoint you.

Some people told me that to legally obtain Helm of Opposite Alighment you need
1) have ToSC installed
2) Kill demonknight on the surface - the first time he appears. Don't ask me how to do this, but he supposedly is wearing the helm at that moment.

That's correct. It's rather hard though: the best way to kill him is probably by succeeding in holding/stunning the demon so that he can't flee. I never tried it myself, but I heard of people who've done it.

Petrarcus ap Bhaal.

I can´t understand you. If I remember correctly, demonknight is the last monster on DT, and he is already in the center of this 'temple' when I explore it. Please explain it further.

When you go on the trip with the dude from Ulgoth's Beard (I believe he's called Roe), you can agree to meet him at DT. Do this, and when you first enter DT, you'll reach the room with normally just one skeleton. Now there are several other tourists there, along with this Roe. Talk to him several times, and eventually the Demon will appear. You can have a fight with him, but he'll flee when he reaches a certain amount of HP. This is what makes it so hard to kill him on the spot.

As I said, I never tried to kill him there, but it should be possible. The question is, do you really want the Helm? And secondly, I don't know if the Soultaker dagger is on the Demon then. It would be a shame if you only had to explore this first room of DT to complete Hurgan Stoneblade's quest!

Petrarcus ap Bhaal.