First, I would point out that trademark does not mean the word is off limits in all situations (they don’t “own” the word), and a video game that came up with the term independently for something only vaguely related would very likely be considered something different enough.
Also, when did they trademark it, and when did it end up in the game (should be determinable from the source archive, I would think)? If it was provably in the game first, they can take a hike.
Of course, even if either of those two things apply (I’m actually curious on the second one), it’s far FAR easier simply to give it a different name, and I don’t think it really matters. Considering that they weren’t utter donkey’s behinds about it (which is sadly somewhat rare these days), that would be the polite thing of us to do - I LIKE rewarding good behaviour.
I would suggest a polite response mentioning that your website is simply a reference point for the material in a non-profit, volunteer-made game with public source code, that nobody intended to infringe anything (or even knew it was trademarked), and we will change it something else as soon as reasonably possible. Commenting on the politeness of the cease-and-desist is optional, but I would - again, rewarding good behaviour is a good thing.
Mind you, I have followed some of this stuff the last few years, so I have a clue or two, but I’m definitely no expert.
Oh, one more thing: defense of trademark is legally required. Trademarks which aren’t sufficiently defended can be lost (see “thermos”, which was once Thermos brand insulated bottle), so sending such a letter when it seems even close to reasonable is highly encouraged - they aren’t just being idiots about it.
From the email, they seem to have it registered independently in the US, the UK, and several other places, so I don’t think “international” is really going to matter much - it’s going to be local everywhere people are playing this game.
