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