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.
Find all secrets
Acquire the legendary Necronomicon.
Claim all areas in the Citadel.
Claim all areas.
Claim all areas in the Wastelands.
Explore all areas.
Unlock all the basic statues.
Use ring of plague to infect the living via chickens.
Gain a million gold.
Sell 20 skulls.
Raise 1000 corpses.
Claim all areas in Burning Rock.
Summon a hero from a hero statue.
Chickenify a dire wolf.
Claim all areas in the Catacombs.
Claim all areas in Alwyn.
Reach level 5.
Raise a horde of 50 undead.
Raise 15 corpses at once.
Slaughter 100 chickens.
Command a single unit to kill 20 enemies.