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.

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.

---- 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.

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THIS IS SUPPOSED TO BE AN INDEPENDENT POST! dammit, GOG!

Then there is the questionable claim From this geocities archive: http://www.reocities.com/hentaihelper/dragwars.htm

It might be true and all, but why isn't the employee named?

Heal does appear in the game. Valar (if i remember his name correctly) starts with the spell. Furthermore, most Healing Potions (including the one obtainable infinite times in the Tars Ruins Underground) cast that spell when used.

Wrath of Mithras is used by at least one enemy.

I don't buy the "too powerful" explanation. Zak's Speed, when stacked, trivializes any encounter that doesn't involve spell casters or fire breathers. Furthermore, there are plenty of items that cast Zak's speed, including some that clearly shouldn't (some bombs, one specific piece of Magic Chain). This means you get access to the spell before the pirate fight (where it is extremely useful) and can later get near-infinite casts thanks to Charger.

Also, Inferno does the same damage as Wrath of Mithras and hits more targets. If Wrath of Mithras were truly overpowered, they could have just reduced the range (but still keep it longer than Inferno).

Also, Light Flash, IIRC, is not any different from Brambles in its effect. (By the way, Brambles is really annoying when enemies use it on you.)

Indeed, various sources talk about enemies casting it on you, perhaps that faq does as well.

As for me, I just cheesed my way through with the gatling bow and the quiver of infinite arrows. I forget the exact names.

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.

Hahahahaha... Unexpected authority bomb!

Disk space seems like a sad reason to have cut the game down, as larger disk-count games were being released around then, but I guess it was a cost factor.