there's a bio here if you want, but i'm just gonna do a recap of the encounters i remember:
(Alex, you're gonna help me flesh this out i hope; Ethuli'en didn't meet Lydian until the party had more than a couple of dealings i think.)
First saw him when he was in his tower (actually now that i think of it, when E. was crowbarred into the story, we were in Lydian's tower chamber right in the middle of something). He asked the party to go find his apprentice who was investigating a Kobold cult (the party had just fought a bunch of Kobolds so we figured it was related) and bring back any information they could find about the Kobold's reasons for attempting to summon the demon prince C'thai'ran. Also what happened to his apprentice, but that was secondary.
After some Kobolds and some Shad'ar Kai Witches who used Crispin's shadow to gain entry to the Kobold cave, we learned that Lydian might know a lot more about C'thai'ran than we thought...
So there was some betrayal involved, some artifact acquired, and a few other things that i can't recall, mainly because the ritual is most important.
Lydian was basically in tune with a bunch of devils from a Shadow Dimension and he was bringing a big 'un to our Plane. Sadly (awesomely) he succeeded.

So C'thai'ran was defeated and sucked us all into the Shadowfell along with Lydian.
this was the end of Part I of the campaign.
In Part II, we met Lydian again (after a bunch of other stuff that isn't related to him).
turns out he was a Vampire Wizard and is soul came back to the Shadowfell when he was plotting his return to our Plane. Lydian was murdered by Fenrak and Ethuli'en broke a piece off Lydian's own coffin and leaped through a wall of fire to drive it through his heart.
Lydian's Flesh turned to ash and his bones dissolved and all that was left was a crystal heart.
After escaping his crumbing tower, we heard a voice from the crystal.
To our delight, Lydian's soul is now trapped in a crystal heart that Ethuli'en keeps in his pants pocket. Currently he is suffering eternal torment.
We're looking forward to how he's involved with the rest of Part II.