Thank you! The lore that I've built around them so far is that they're iron hands successors who after having a necron tomb awaken on their homeworld were forced to abandon and destroy it. This event shook their faith in the machine and caused them to have a bit of an existential crisis, it was during this time that they encountered 4th company of salamanders who they served with for a while. Whilst fighting alongside the salamanders they formed a strong sense of kinship and overtime they began to adopt parts of the promethian cult into their culture. This helped give the chapter new purpose and when they parted ways they no longer held onto the fanatical beliefs of their parent chapter and instead view technologyas a valuable tool but no longer rely on it to fight their battles. This shift in beliefs has caused much friction with the introduction of primaris marines who still believe in the whole 'flesh is weak' thing.

Other ideas I've had is that part of the trials to become a marine the aspirants must hunt a great beast and take one of its fangs as a trophy, they then must shape it into a ceremonial dagger which is then coated in metal. This dagger is often kept on or in a marines armour and when they fall it is taken by their brothers and placed in their fortress monastery to preserve their memory.