Here is the full list of all 24 Undead Horde achievements.
Chickenify a dire wolf.
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.
Find all secrets
Unlock all the basic statues.
Acquire the legendary Necronomicon.
Gain a million gold.
Claim all areas in the Wastelands.
Claim all areas in Burning Rock.
Raise a horde of 50 undead.
Command a single unit to kill 20 enemies.
Sell 20 skulls.
Raise 1000 corpses.
Claim all areas in the Catacombs.
Summon a hero from a hero statue.
Claim all areas in Alwyn.
Slaughter 100 chickens.
Reach level 5.
Raise 15 corpses at once.