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