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