The Shadow Hunters

So this is just a bit of background information on my chapter: The Shadow Hunters.
They are a recently formed chapter, situated of the eastern fringe of the galaxy. They have some very specialised close combatants but predominately excel in ranged warfare. This chapter are constantly attacked by Ork raiders called the 'Ead Smasha's'. Chaos is a constant threat too.

So any ideas to say when they were formed?

Sky-Slasher

Maybe mention why they were formed - to combat the orks there maybe?

Ask ChangerOfWays when he's on.

They were formed to combat chaos.

Hmmmm.... are you using the Blood Angels codex? If so you could say they formed a different chapter, like they used to be a company but their commander was swayed/killed by chaos so they decided to go on a penance/vengeance crusade and stayed in that sector, waging war until they have atoned for their sins/killed every last mutherfucking chaos scum

The Black Templars codex but they are not successors of them.

Hmmm.... you could pretty work the storyline(s) that I made up and apply it to any chapter.... or even just do it for the whole chapter, and they refuse to move until they kill every last chaos/finally atone for their chapter master's betrayal, by killing every last chaos xD