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