Iron Scorpions and their successors, other Chapters that have recovered from a heresy.
Imperium of Man
The Thirteen Commandaments of Iron are a litany of 13 rites of honour that are used instead, or in accordance to the Codex Astartes, as a rite to follow in order to prevent a heresy.
They were written by Iron Warden Urexus Banesword, after seeing that the Codex written by Robute Guilliman could possibly not be enough to ensure preservation from the taint of chaos. Today it is mostly in use by chapters that have experienced, or have dwindled at the brink of a heresy (though not all of them) to ensure that it would not happen again.
The most rigorous followers of this passage are the Space Marines of the Iron Scorpions chapter.
The Thirteen Commandaments have lately been expanded into a book of about 2000 pages, which includes hundreds of inserts about successful tactics and war methods, some of which are just expanded and refined versions of the battle methods found in the Codex Astartes, and also includes passages detailing military organisation and the moral behaviour expected of Space Marine Chapter. It also has an entire section dedicated to the Thirteen Commandaments of Iron, and outlines further the proper way in which this command must be followed.
In short, this tome is basically an expanded and refined version of the Codex Astartes.
Revere the Emperor above all, serve him and protect all that serve in his name.
Any who follow his ways like us are deserving of our lives, and we must spread the meaning of our purpose to other marines who have lost their way off the path of his light.
Fear is a promise of defeat.
Fear instils the sense of confidence and success in your foe, and increases their morale; This is not what we must achieve.
Make every blow your strongest and every shot strike true
Your foe is bound to be skilled, ensure that you are always the most skilled opponent on the field.
Alliance only breeds corruption, only the Emperor's finest may be trusted
Alliance is only a breeding ground for grudges against others, and equality spells petty arguments of popularity between the leaders. Only trust the strong minded and purest servants of the Emperor.
Show the enemy no mercy, no respect and no pity, death is the only gift for them
Respect the Iron warden and all those under his command, for they know the way around the terrors you will be facing
Arrogance is an offence equal to that of heresy, for only the Emperor is perfect
The only disraction that may cloud your mind is your duty to the Emperor, hate of the Imperium's enemies, and perfection of the art of war.
Finish what fallen brothers have failed to accomplish
Question change until all knowledge of it is gained, for the old ways have ensured our victory since our rebirth, new ways can only be prefected through error and failure - And failure is never an option.
Be tempted by nothing blasphemous, for even the slightest hint of heresy will spread.
Avenge all deaths of your brothers second only to the death of Rogal Dorn
Obey these Thirteen Commandaments down to even the last word.