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