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