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