Not a lawyer. Anyhowwwwww...
Libris Mortis = (EN) Book of the dead.
Note that does not translate into "Book of Undead" as per the 5e book.
>US Copyright laws on names:
The Copyright Act states that names, titles, short phrases and expressions cannot be registered in the U.S. Copyright Office.
>Trademark laws on names:
1) Trademarks are registered to protect the goodwill of companies and individuals selling their goods and services, and to help people properly identify the source of goods or services.
2) For a person's name to be trademarked, it must already be distinctive in the eyes of the public, so that when people hear the name they think of the goods or services offered by that person.
So if WotC wants to sue you, well...
Part 1) They can always choose to throw you a Cease & Desist, if they bothered to that is, and in many case people immediately cave in when they get one, EVEN THOUGH the C&D in question might not have a leg to stand on.
Part 2) If you don't just roll over though, in that case it is WotC's responsibility to PROVE
that what you did a) Infringe upon their rights and/or b) Bring harms on their brandname. - In both cases, they must FIRST provide proof that the term "Libris Mortis" is uniquely theirs, that they would be the very first thing everybody think of when you mention it.
Calling yourselves "Libris Mortis"
is likely okay from WotC, since it is a bunch of words without brand significance.
Calling yourselves "Libris Mortis: The Book of Undead"
though... You might get a C&D if you ever get big enough.
Calling yourselves "Libris Mortis"
and refers to yourselves as being heavily influenced by D&D... That miiiiiiight be cutting it close? Though I am not completely sure there. (Again, NOT a lawyer.)
On the other hand, a more specific google search re: +"libris mortis" +band
, there are a few other hits:One music trackOne bandOne novel
It can be seen as proof that shows that the word "libris mortis" by itself isn't completely unique enough? Maybe?
As for having another band with the same name, well, they have to be able to make the same claim that they have some form of "brand significance" that, seeing they only have 1 song... Even if they ever bothered, or even have enough $$$, to hire a lawyer, they likely cannot hold claim to another continent.
So, I would say "go for it".
They can certainly sue you, but "suing" and "making a case" are two different things altogether. (Just see how Hollywood is misusing C&D on online video sites like Youtube & stuffs... )
That's also based on whether they would be bothered enough to sue too.
(AGAIN & AGAIN, NOT A LAWYER. Can't make any guarantee.)
If you actually get to a point though, by then you'd need to find a lawyer for various stuffs anyhow. If you're just starting out maybe its not something to worry about... Dunno?
As for other words, Google is your friend.
Magic Missile depends,
Chromatic Dragons nada,
Elminster f__k I think someone's actually been sued before?
Anything that had appeared in at least 3 different & unrelated brands were probably safe.
(Eg. Frost Giants. Skyrim, D&D, Marvel. No problemo!
Fire Dragons. It's actually harder to find something that didn't
Calling your Fire Dragon a "Chromatic Dragon" would be against D&D copyright since only D&D use the term.)
Also look at other bands who also claim D&D influence. - There are at least a handful I've heard mentioned before. Never followed that though. - Go check out their stuffs to see how they deal with the issue.