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