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