"The king is dead. And the queen is in trouble. She has chosen you to find the fabled heartstone, which is buried somewhere deep in the Mountains of Ice.
Is the gem in the hands of the corrupt Master Thief? Has he harnessed its powers for his own vile purposes? Or does it lie unguarded in the frozen ranges to the north, which teem with horrible creatures?"
But that's not really the focus here & instead I want to speak about one of the most dangerous NPC bastards that your party of adventures can encounter. I'm speaking of Kelek the Cruel & XL1 The Quest For the Heart Stone would take place long before the Season 1 Dungeons & Dragons cartoon episode "Valley of The Unicorns". During XL1: "Quest for the Heartstone Kelek is a mere 7th level wizard of some renowned.
Between the events of XL1: "Quest for the Heartstone & the "Valley Of The Unicorns" cartoon episode Kelek gained Venger as a demi god demonic patron.
Yup that's right I said demi god DCC style patron. Over & over again the evil Venger survived being 'destroyed' by the Dungeon Master's Young Ones. Kelek was completely obsessed with gaining immortality & in Venger's service went to tenth or beyond level as a wizard. He has moved from Mystara to the Realms where he helps Warduke during Venger's campaign of terror to bring the Realms to heel. Afterward he would join Warduke on Greyhawk during the Greyhawk Wars. Possibly in the service of Luz after escaping the prison of Venger from the "Valley of The Unicorns". In Luz's service he becomes even more dangerous making pacts with the 'unnameable' Venger's Abyssal patron. Helping broker deals on Warduke's behalf for rebuilding the Horned Society, and was instrumental in eventually helping Warduke become a Hierarch himself. This didn't take long & he cut infernal & Abyssial deals towards the ultimate goal of lichdom.
By this time Kelek had learned to stay to the background & finally after helping Warduke build his base of power & continue his existence. Kelek gained lichdom & immortality of a sort. He's run afoul of Gort The Rogue once but that encountered led to a hasty retreat back to his own backwater planar base. He's really maintained a low profile because he still fears the wraith of Venger. There's a lot of bad blood between him & his old patron. Kelek the Cruel has quietly been building a large army of gnolls, orcs, & other humanoids on the far end of a Prime Material plane.
He often now employs parties of adventurers to recover relics, occult treasures, scrolls, etc for him. Kelek pays on time but often takes a swift revenge on any who cross his path with his own assassins. Rumors are spoken of him building his own power base with an aim of returning to his own home plane of Mystara for a bid to take over a nation or two. The ultimate goal could be revenge against Venger at some point.