How to get Sith Lord Achievement without doing DS playthrough

First you have to find your Kotor2 file. It is mostly installed in your random disc file.

My Computer/Disc C, D or anything you have named and where you installed your steam etc/Program files (x86 if you are using 64 bit OS)/Steam/Steamapps/common\Knights of the Old Republic II

Now double-click on that ini file.

Now that you have opened swkotor2.ini search for the line called: CurSithLord. You can also press ctrl+f and paste this line. After you found CurSithLord line, change the current number (mostly 3 if you have completed the game) to 4 and save. it should be like that:

The current numbers in order are:

0=Darth Sion
1=I have no idea
2=Darth Nihilus
3=Darth Traya
4=Your Current PC. (Playable character)

Number 4 is automatically selected after you completed the game in Dark side.

Congratulations. You have received for some reason achievement and your PC is showed in the main menu. Your Character shows how badass he/she is doesn't it?