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