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