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