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