Here is the full list of all 20 Unholy Heights achievements.
Recruit a top tier Cheepy
Sucker a top tier Undead into moving in
Conjure a top tier Elemental into your building
Let a top tier Abyssal darken your doorstep
Make room for a top tier Demi-Human
Invite a top tier Demon into your home
Domesticate a top tier Chimera
Add a second floor to your building
Add a third floor to your building
Add a fourth floor to your building
Complete the main quest
Let none die under your watch
Evict none throughout your campaign
Even love can bloom on the battlefield
Witness the miracle of monster reproduction
Have a resident skip out on paying their rent
Have an unemployed resident find a new job
Witness a resident swearing to avenge the death of their child.
Fill every possible vacancy
Defeat the legendary hero at the first opportunity