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