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