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