Here is the full list of all 24 Undead Horde achievements.
Start a New Era.
Find and complete all quests.
Claim all areas in the Citadel.
Claim all areas.
Explore all areas.
Claim all areas in the Wastelands.
Find all secrets
Unlock all the basic statues.
Gain a million gold.
Acquire the legendary Necronomicon.
Raise a horde of 50 undead.
Use ring of plague to infect the living via chickens.
Sell 20 skulls.
Raise 1000 corpses.
Claim all areas in Burning Rock.
Claim all areas in the Catacombs.
Claim all areas in Alwyn.
Kill six humans with a single scythe sweep.
Summon a hero from a hero statue.
Chickenify a dire wolf.
Reach level 5.
Raise 15 corpses at once.
Slaughter 100 chickens.
Command a single unit to kill 20 enemies.