D&D 5E Undead Dragon-Fiend

[Shadowcortax]

So I'm trying to help my DM (who also happens to be my fiancee) convert an old PC to a Monster. Here's some back story on the PC. The PC was a NG Male Half-Orc Paladin of Torm with Oath of Devotion. He was part of a party that played RoT and at the end of the fight a party member, who happened to be a CE Female Warlock of the Old One who had been at odds with him the whole campaign, shoved him into Tiamat's claws as she was sucked back down into the portal. So the PC spent the next few months being tortured by Tiamat, stripped of all his levels, pulled apart, reformed, pulled apart again, told his deity was dead, and this was on repeat until he broke. Tiamat then remade him as a White Dragon-Fiend and sent him to train in Orcus' undead fields. After being broken and remade he is utterly devoted to Tiamat, especially after gaining a new body and new weapons and armor. While training in the undead fields he began taking on some of the properties of undead gaining abilities similar to a lich/death knight. So after a few years Tiamat calls him in and sends him to the Prime Material Plane to hunt down his old party members and one of the members of the current party as well. She provided him with armor, a shield, a sword and a mount.
I finished his new stats, his sword, Shield, and armor stats, but I'm having a bit of trouble with his mount.
He has Dragon spawn (with half the gifts), abishi, and death knight stats with a few other goodies thrown in,
His sword, shield and armor use exotic materials and are kinda overpowered on him.
For the mount I'm thinking a Shadow Dragon disciple of Ashardlon that became a Dracolich or maybe took on some Dracolich like abilities. Any suggestions?

 

[aco175]

Flying carpet that can teleport.

A demon lion thing with fire like a nightmare but stronger.

If he is meant to be a solo against the PCs, he can have all sorts of things and still be killed rather quickly. Make sure he has lair actions, maybe his flying carpet acts as his lair.

 

[Shadowcortax]

More looking for stats for the thing. Shadow Dragon disciple of Ashardlon that became a Dracolich or maybe took on some Dracolich like abilities.

 

[Eltab]

A one-headed Chimera, perhaps? Or re-fluff a Peryton.
Make sure his saddle includes a seat belt, or the new PCs will figure out to taunt the mount until it pursues them through a barrel loop.