I've been thinking lately about how the Overlord novels/manga/anime are so clearly based on 3e/3.5e/d20/whatever, yet were still commercially published--and, as far as I'm aware, suffered no legal action from Wizards of the Coast. Much of the "mechanics" of the series (at least from what I've seen) are entirely possible within the parts of d20 that are covered by OGL.
An example: at Overlord's spell magic arrow
This appears to be a clear copy of d20's magic missile. It's a 1st-tier spell, equivalent to a 1st-level spell, and it launches an unavoidable bolt of non-elemental (equivalent to force damage, or not having an energy type) magic that deals a small amount of damage and cannot be blocked by normal means. The spell can also create multiple bolts if cast at a higher tier/level, just like how magic missile would (depending on what exactly the Overlord wiki means by this, possibly similar to the Spell Points variant rule, also open content)
By my reading of the OGL 1.(e), "Product Identity" (which, as per section 7, must be agreed to not have any of the following done with it, from 1.(f): "Distribute, copy, edit, format, modify, translate and otherwise create Derivative Material of Open Game Content"; 1.(b) defines Derivative Material as "copyrighted material including derivative works and translations (including into other computer languages), potation, modification, correction, addition, extension, upgrade, improvement, compilation, abridgment or other form in which an existing work may be recast, transformed or adapted") includes both the "spell" magic missile and the "magical...effect" produced by magic missile, as well as any modification or adaption thereof.
Mechanical similarity looks like it's derivative
The effects of magic arrow are clearly derivative of magic missile. But the specifics, such as dealing 1d4+1 damage (or what 1d4+1 damage even translates to, beyond rarely being enough to kill a target with one shot), or having a range of 100 ft. + 10 ft. per caster level, or any of those details which pertain to actual d20 mechanics, do not seem to be mentioned in Overlord.
This brings me back to the question, which is more general than just that single spell.
How is it that Overlord's use of things which seem like they ought to be forbidden due to being considered WotC's "Product Identity", is actually okay?
- Is it because Overlord isn't a game (in which case, where are exceptions like this stated in the OGL?
- Does it have to do with the fact that the above details are generalized into a written/drawn form?)?
- Is it because magic missile isn't explicitly designated as Product Identity beyond the proper name of itself as a spell (in which case, what about spells like sleep and animate dead, which Overlord keeps the names of, or elements such as "troll" creatures with high strength and what amount to d20's Scent/Regeneration abilities?)?
- Or is it something else entirely?