I have talked about Tiamat a lot here. Not just the D&D concept of her, but the ancient myths and what she means in my world. She is the current "Big bad" in my kids 3rd edition game. They began as just a small group looking into researching dragons. In the process they discovered the rise of the old Cult of the Dragon (I am using what appeared in Dragon Magazine before it was went over to the Forgotten Realms) into a new threat. The feel the only way to stop this evil from taking over the world is go to the source.
The twist I am planning is the artifacts I am having my kids gather up to summon Tiamat are also just what she needs to come into this world to rule it. I am using ideas from the old Doctor Who serial "The Key of Time" and the Come Endless Darkness book by Gary Gygax (more on that book later). They have gathered up all the relics they need; and these are true relics, they are the remains of dragons that are now "saints" in Tiamat's evil pantheon. They are going to summon her using these relics and a few other items. Then there is going to be a big no-holds-barred fight on the Dragon Isles.
There are some parallels here with my last campaign/game "The Dragon and the Phoenix", but this one should be a lot bloodier.
Here are some of my relevant posts on Tiamat
Tharizdun is closer to the classical idea of "The Devil" than Tiamat is. Though I do recall at one point thinking that Orcus was the son of Tiamat and Tharizdun. Not keeping that, but I might revisit it one day.
Unlike many of the other creatures I have posted here Tharizdun was created whole cloth by Gary Gygax and expanded on in later books. He is the main bad guy in the Gord the Rogue books including Come Endless Darkness (he is on the cover in fact). In my games he is god chained at the bottom of Hell and Asmodeus is his jailer still. Though the millennia of Tharizdun whispering in his ear it is hard to say how much of the original Asmodeus is actually left.
He is hinted at in the T1-4 modules and then bits and pieces in S4 and WG4. I think it would be very interesting to do the entire GDQ series under AD&D but instead of Lolth being the big bad, make her an unwitting pawn of Tharizidun. Get the Shard of Pure Evil (from 4th Edition) so he can escape his prison and destroy the world.
Sure it is an awful lot like my Tiamat arc above. But it works, and the stakes would be much higher.
Can't wait for my kids to fight these two!