Shackled souls are unfortunate mortals that spent a long time trapped within one of the Nine Hells. Most rangers choose this archetype after being imprisoned in one of the hells and breaking the infernal soul chains that shackled them to the hells. These chains are made of unbreakable solidified hellfire and were created by the archdevil Mephistopheles to hold his most dangerous and valuable rivals within his realm, but were later mass produced and given to the other archdevils to imprison souls. How you ended up as a prisoner to an archdevil and broke your eternal shackles is your tale to tell, but those shackles will always be a burden and reminder of the past.
Art by Tomasz Chistowski on Artstation