IF YOU STAT IT, THEY WILL KILL IT
However, if you decrypt the MON1CHA.DAX monster datafile, stats for Manshoon exist. Here are the stats browsed with Monster Modder (included with Gold Box Companion).
There's also a combat icon in CPIC1.DAX that is not used in the game.
Now we just have to put him somewhere into the game.
Remember the boss fight against Kalistes? There are distant monsters on three sides of the characters. This is a good scenario to test a spellcasting villain.
The monster count for this encounter is: 1 x Kalistes (monster record 13 = $0D), 7 x Drow Champion (37 = $25), 8 x Drow Wizard (40 = $28), 5 x Pet of Kalistes (7 = $07). Monster record numbers can be seen with Monster Modder.
Gold Box game map triggers and encounters are stored in ECL-files.
After decrypting ECL1.DAX and searching for some related strings (I used "HOW DARE YOU" from the screenshot above), this encounter can be traced to record 82 of the file.
At offset $1AAE there are following "load monster" commands:
Now if we change those to:
Should result in total 6 x Manshoons (well, he likes to clone himself), replacing all previous monsters. Manshoon's monster record is 85 = $55.
At $1A65 there's a "setup monster" command which displays the picture at the the "HOW DARE YOU?" screenshot. We'll change the $0D (Kalistes' monster record) to $55 (Manshoon's).
And after rebuilding the ECL1.DAX file we get...
If you want to try your luck against 6 Manshoons, download manshoon.zip. Remember to backup your own datafiles / saves!
Do not expect anything extraordinary as it still uses the Gold Box AI and is mostly spamming Delayed Blast Fireballs. You can try changing some of his spells with the Monster Modder.