Oh, awesome then.

I love the secretive aspect of him and it's one of the greatest reasons I chose him. Below I've detailed the campaign, most of this is a mystery to my players but there have been subtle hints. I've adjusted it to include the new information about Tharizdun.

And trust me when I say, I didn't change much at all. The gems existed in my campaign (although, originally as a different form but adjusted to fit the lore), the concept of Tharizdun already existed (I had orignally called him the World Eater which is also one of his actual aliases) and my cult was conveniently named very close to one of his.

They don't use his symbol though.

Thanks for the info so far!

Campaign:
Spoiler
Show

Around 800 years ago a great war broke out between a major Tiefling and Dragonborn kingdom. This was a brutal war, the Dragonborn prevailed through skill while the Tieflings developed advanced weaponary (including spells).

Things went on for about 30 years, intense warfare, until the Dragonborn were able to recruit allies after gaining intel that the Tiefling kingdom was ready to release an incredibly devastating weapon which would proove harmful to not only them but whole other nations.

The new allies stormed and conquered the Tiefling home and in a last ditch attempt the Tieflings unleashed the weapon on themselves.

This weapon, unwittingly, was (now that I know) Tharizdun. The Tieflings had enough intel to know that gathering the 333 crystals released a tremendous power but they had no idea what.

Tharizdun broke into the mortal world and existed for all of 30 seconds.

In the first 10 seconds he wipes the Tiefling's city off the map, killing everyone inside.

After 20 seconds he had devoured half the continent and burrowed down into the crust, hungry to consume the world from the inside out.

After 30 seconds he consumed the core of the world and the planet began to shatter.

Gods, both good and evil, intervened the moment Tharizdun consumed the core and turned it around on him, trapping him inside and making the core the 334th crystal. They then took Tharizdun's chains and bound the world together with them to prevent it's destruction (in my campaign, the chains are made of pure, arcane energy) and used the 333 gems to create crystals on each planet fragment which would draw natural arcane energy to power the gems.

Many nations had to sacrifice this energy up front though and so there were mass ritual sacrifices of those with strong arcane powers. This prevented communication between nations for a long time (most which had no clue what happened) and resulted in a large loss of information as the Tiefling city had been destroyed.

Fearing that Tharizdun would be sought after by foolish mortals yet not trusting even their own followers, the gods put him in orbit as the red moon. Far from the reach of any mortal.

Now, 800 years in the future, a group called the Brotherhood of the Red Moon (now, officially an offshoot of the Scarlet Brotherhood thanks to wiki) is following orders from Tharizdun's avatar, Shothragot who I need to learn more about. They are focusing on collecting the gems under a false promise that this will help reunite the planet back to it's previous form.

Beyond this they know nothing but they're willing to go to extreme lengths for the gems. The party has two (without realising it) which, unless properly contained, exude horrible energy, warping everything around them into chaos.