Edit: Oops! My error. Apparently the game features BOTH a Rage of Mithras AND a Wrath of Mithras.
Indeed, no one seems to have posted any info on where to find it.
Leaving my erroneous guesswork from my original reply below.
I played this game over a decade ago and that spell was nowhere to be found.
By the way, I determined through save hacking that every spell in the manual is implemented and works as described. Whether they can be found is another matter.
According to this walkthrough: http://www.classicgaming.cc/pc/dragonwars/walkthrough.php
The (mistranslated? copy error? typo?) Rage of Mithras can be acquired from Berengaria at the Mud Toad tavern after escaping the Phoebus Dungeon.
THIS IS SUPPOSED TO BE AN INDEPENDENT POST! dammit, GOG!
Then there is the questionable claim
Yes, the spells "Heal," "Light Flash," "Wrath of Mithras," and "Prison" are not found anywhere in the game! A former Interplay employee told me that they were taken out after playtesting showed they were too powerful.
From this geocities archive: http://www.reocities.com/hentaihelper/dragwars.htm
It might be true and all, but why isn't the employee named?
I'll have to check the source code to see where those are found. There were five maps that were cut from the game due to lack of disk space when I wrote the game in 1988.