Obligatory IANAL, but I think it's the text itself that's copyrighteable. So you can definitely use the same die rolls or whatever as long as you significantly paraphrase.

Spells and classes names cannot be used if they are trademarked, right? But if WotC didn't trademark "fireball", "paladin" or "hobgoblin", you can totally use them.

So you can't have a spell called "arcane missile" that does 1d4 damage every 2 levels of the caster? Can you really copyright the concept of an ability that deals (level // 2) * 1d4 damage to an enemy?

My understanding is that this is where things get tricky: realistically WotC probably can't go after anyone that creates an ability that is mechanically identical to DnD ability. Things like spell names, might be, but "fireball" is far too general to be copyrighted. But what about a spell called "fireball" that also has the same mechanics? That's where things can get complicated.

So what if they just doubled all HP values and damage values, and changed any names that are not unambiguously generic ("invisibility", "fireball", etc.) ? Damage values no longer overlap, and spell names are different. On what basis does WotC have to claim infringement?

> So you can't have a spell called "arcane missile" that does 1d4 damage every 2 levels of the caster?

I think you can. I don't think copyright covers gameplay mechanisms, it just covers the actual text of the rules. But IANAL.

I think for a single spell there's fairly little you'd need to modify (just the actual text of the description, and changing unique names). Not even changing damage dice.

The problem is making a whole spellbook with a one-to-one correspondence to WoTC's spellbook, even if each of those entries has been modified. The spellbook is more than just a bunch of separate spells - it's a collection carefully designed for balanced gameplay and has protection in itself. See [1] for one way that "compilations" can be treated as having more rights.

The EU also allows copyright of databases [2], and I think the spell list meets the criteria.

And don't forget you'd also be doing the same thing with races, classes, equipment, monsters, conditions, deities, feats, blah blah blah. Put together I think it's pretty likely to be infringing somewhere! Certainly enough that a lawsuit isn't getting dismissed.

[1] https://www.copyright.gov/circs/circ14.pdf [2] https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX...

Not a lawyer, but my understanding last I looked into it was that names and rules cannot be copyrighted (names can be trademarked though). In very limited cases a novel mechanic could be patented (famously MTG 'tapping' of cards). The issue overall is that no one wants to risk it in court, so there's no recent precedent to take inspiration from.

> Not a lawyer, but my understanding last I looked into it was that names and rules cannot be copyrighted (names can be trademarked though)

That's what the person you're replying to wrote.

Interestingly, I got a conclusive answer to this question when working at S2 on Heroes of Newerth.

The CEO accidentally CC'ed me (he meant to CC Sean, not Shawn) on a letter from Valve's lawyer threatening us with legal action for copying Beastmaster's mechanics. They had a side-by-side of ours (https://hon.fandom.com/wiki/Tundra) vs theirs (https://dota2.fandom.com/wiki/Beastmaster), and pointed out that the mechanics were identical.

It was very surprising. So the answer is "Maybe not, but that won't stop companies from threatening to sue you." Or "Maybe it is, and companies will threaten to sue you." No one really knows, since it hasn't been tested in court.