Back in 1979 I had a problem. As a new DM I was desperate to have a gritty, realistic campaign full of sturm und drang (yeah, I was one of those DMs) with Big Ideas and lots of Drama. But, well, everyone in my party was only 12, like me, so the characters had names like BadAxe and Joey Longhair. This didn't fit my Ideas About the Campaign! Why weren't they using the 9 page glossary of common names and name elements, by race? Huh?!
I like to think my first real insight about being a good DM happened while I was wrestling with this problem. If the players were unwilling to change something like that and it wasn't, oh, unbalancing the campaign, why not change the campaign so that it fit, anyway?
That is why my campaigns have the concept of True Names. Certain creatures (mainly Humans, demi-humans, humanoids, etc.) are born and first named that is their true name. A spell caster that knows the true name of a target and uses that name in spell casting has a greater power to affect the target. For this reason, most people have what is called a 'Day Name' or 'Friend Name'. Some societies take the idea so far as to have a true name, usually known only to the person and their parents, a 'house name' which is used only be their immediate family, and a Day Name used by everyone else. It is also fairly common for someone to change their Day Name at key moments in their life, such as leaving home, becoming a master at a craft, etc.
This explains why one wizard may be named Altrazar and the one next to him is Firewalker - Altrazar is probably the Day Name his mother gave him while Firewalker is one taken when he left his apprenticeship. The great thing is, they both make sense in universe.
What effect does knowing someone's true name have? Well, I have a small section on this in my OSR supplement Mage Guild, that has a handful of ideas, but here is one suggestion: if you know the true name of your target they make all saves vs. your spells at -4 and any magic resistance is cut in half. Mage Guild also has the 8th level spell Naming which allows high level magic-users to really ruin your day if they know your true name!
If you use a mechanic like this then I suggest that you also make finding a true name tough - asking a Charmed target their true name gives them an immediate save at +4; if using ESP the target gets a saving throw to keep their own true name unavailable to the caster; etc. Learning the true name of a Big Bad can be a major quest worthy of a long series of adventures.
On a side naote, years later when Glen Cook's novel The White Rose came out my old friend Eric called me up,
"That is so cool! We did the same thing in your campaign in '83!"
That was a good phone call.