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