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