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 all secrets
Find and complete all quests.
Claim all areas in the Citadel.
Claim all areas.
Explore all areas.
Unlock all the basic statues.
Acquire the legendary Necronomicon.
Claim all areas in the Wastelands.
Gain a million gold.
Raise 1000 corpses.
Claim all areas in Burning Rock.
Sell 20 skulls.
Summon a hero from a hero statue.
Claim all areas in the Catacombs.
Claim all areas in Alwyn.
Reach level 5.
Slaughter 100 chickens.
Raise a horde of 50 undead.
Command a single unit to kill 20 enemies.
Raise 15 corpses at once.