Here is the full list of all 30 Herald of Havoc achievements.
Beat wave 10 on Endless mode: Black Fortress
Beat wave 10 on Endless mode: Arena
Beat wave 10 on Endless mode: Proving grounds
Kill every enemy and find every secret of episode 3
Finish episode 3
Convert 20 enemies at the same time with the special ability of the Soul Reaper
Resurect a Pyro with the Soul Reaper
Find the Super Secret in E1M4
Beat E3M1 - Hide and Seek in Havoc difficulty
in E3M1 - Hide and Seek... kill everyone in the machine room
Kill every enemy and find every secret of episode 2
Finish episode 2
Kill 12 enemies under one second with the Runeblade
Find a way to enter the blue key room in E2M6 - Sealed Secrets
Control 20 or more zombies at once with the Soul Reaper
Find all secrets in E2M5 - Searing Sun
Have Akimbo and HAVOC powerups at the same time
Fire the Shock Rifle and make the lightning bounce out of 12 enemies at once
Finish episode 1
Kill every enemies and find every secrets of episode 1
Find the super secret in E1M6
Kill an enemy by electrifying a pool of water with the Shock Rifle
Finish E1M6: Ghost town, without getting damaged by anything in any difficulty (damage fully absorbed by armor doesn't count as damage)
Find the secret menu
In the blue key room of E1M5 - Halls of Knowledge, kill the Metal Colossus and the Minotaur without opening any of the two doors, on veteran or Havoc difficulty
Beat E1M3 - Secret Weapon... without collecting any weapons... (only use Runeblade and Blaster)
Beat E1M1 - Inflitration under 30 seconds
Fire the Shock Rifle at a dead body to use it as a conductor in order to kill an enemy
Beat E1M1 in Havoc difficulty
Burn 6 enemies at once with the Blaster's secondary fire