Thelryon is a character from Divine Divinity (minor spoilers for Divine Divinity behind the cut)
In the first village from Divine Divinity you run into a mage who has been possessed by the evil mage Thelryon Hashnitor. To save him you need to find Thelryon's grave, ressurrect him and re-kill him. So when I first played this game and ran into nice and harmless village doctor THELRYON HASHNITOR my first reaction was "Okay, this guy is definitely evil. Like, really evil. Stop telling me he's harmless. Also, Cyseal's undead plague. You can't tell me he has nothing to do with that. He's evil!"
(Spoilers for Original Sin)
has been around for a while as well. In Divine Divinity, you could annoy him by moving flowers from his wife's grave to his. He'd then come out and murder you because he's lv 5 and you just started. You could also find him in his cellar, where once again, murder. He's also a boss in Flames of Vengeance. For a dead guy, he's really been around.
The teleporter pyramids and Zixzax appear in all the games. The other mainstay, Zandalor, hasn't shown himself so far.
(Haven't finished the game yet, so that's speculation, spoilers for Luculla Mines)
Those letters mention a wizard referred to as Z.
A minor thing I just remembered: The Magical Mishaps book have been in most games, with the same content even. They seem to be really popular in Rivellon.