The anti-magic people are in a town named Zigur, it's found around the other side of that mountain range that Derth is close to. The Antimagic quest is detrimental for most players unless they're really jonesing for a different gameplay challenge, so you may want to try repairing your save game first...
One possible workaround would be to
back up your save, restart the same character (suicide until no lives left then 'restart with same character'), get to the world map with that character, then save and exit the game. Now, open the file
C:\Documents and Settings\YourUserName\T-Engine\4.0\tome\save\YourPlayerName\zone-wilderness.teaz (or C:\Users\YourUserName\...etc on Vista/7) with the program
7-zip (Open As Archive), extract the file named something like
mod.class.Encounter-0x0ae94328 which is equal to 1,165 bytes (packed size around 677 to 679) from that savefile. Stick the extracted file on the desktop or somewhere you won't lose it
Now you can close that zone-wilderness.teaz file and open your backup of the high-lvl save where you saved Derth. Restore your backup into the original save folder (probably best to delete the temporary newbie save first), and finally use 7-zip to import that extracted mod.class.blahblah file back into your higher-lvl save's zone-wilderness.teaz file, deleting or overwriting the other ~679 packed size one.
There's no guarantee these instructions are of any use to you as that value after "mod.class.Encounter" (which is different for every character but may be the same between separate restarts of the *same* character) may be referenced elsewhere that will stop this kind of 'npc replacement' method from working, or there may be quest info held elsewhere in the file that would have to be edited, but if you feel adventurous then you may as well give it a shot
Alternatively you can zip up your entire save file and post it on
http://www.mediafire.com or
http://www.megaupload.com and then give the link here so someone else can investigate the issue for you.
