Here is the full list of all 20 Unholy Heights achievements.
Add a second floor to your building
Witness the miracle of monster reproduction
Even love can bloom on the battlefield
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 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
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