Here is the full list of all 24 Undead Horde achievements.
Kill six humans with a single scythe sweep.
Use ring of plague to infect the living via chickens.
Chickenify a dire wolf.
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.
Sell 20 skulls.
Claim all areas in Burning Rock.
Raise 1000 corpses.
Claim all areas in the Catacombs.
Summon a hero from a hero statue.
Claim all areas in Alwyn.
Raise a horde of 50 undead.
Reach level 5.
Slaughter 100 chickens.
Raise 15 corpses at once.
Command a single unit to kill 20 enemies.