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
Beat E3M1 - Hide and Seek in Havoc difficulty
Resurect a Pyro with the Soul Reaper
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 every enemies and find every secrets of episode 1
Find a way to enter the blue key room in E2M6 - Sealed Secrets
Find all secrets in E2M5 - Searing Sun
Kill an enemy by electrifying a pool of water with the Shock Rifle
Fire the Shock Rifle and make the lightning bounce out of 12 enemies at once
Have Akimbo and HAVOC powerups at the same time
Finish episode 1
Kill 12 enemies under one second with the Runeblade
Finish E1M6: Ghost town, without getting damaged by anything in any difficulty (damage fully absorbed by armor doesn't count as damage)
Control 20 or more zombies at once with the Soul Reaper
Find the super secret in E1M6
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
Find the Super Secret in E1M4
Beat E1M3 - Secret Weapon... without collecting any weapons... (only use Runeblade and Blaster)
Fire the Shock Rifle at a dead body to use it as a conductor in order to kill an enemy
Beat E1M1 - Inflitration under 30 seconds
Beat E1M1 in Havoc difficulty
Burn 6 enemies at once with the Blaster's secondary fire