No Prep Module: Assaulting The Undead City (Patreon)

https://www.kassoon.com/dnd/adventure/no-prep-module-undead-city-assault/ 

Vrondiss Umbyrphrael was a blue dragon that took control of the city of Didsleon after a destructive siege that left it partially ruined. At first the dragon allowed the city to continue as normal under his rule (after a large increase in taxes), but over time he began to feel paranoid and exposed about both his newfound citizens and the other nations that  could threaten his rule. The dragon sought ever-greater defenses and  power, and eventually discovered a solution: immortality. Using the souls of his own citizens as fuel for his rituals, Vrondiss transformed himself into a powerful dracolich.  Now the city of Didsleon has fallen even deeper into ruin as citizens  are transformed into eternal undead to become the perfect servants,  their souls torn away as fuel for the dracolich's phylactery.

https://www.kassoon.com/dnd/adventure/no-prep-module-undead-city-assault/