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