I have yet to complete any weapon side-mission chain, but I noticed that, after doing five missions for any weapon, the weapon screen pops up showing some runic writings appearing.
Is there any bonus my weapons gain at this point? If so, what is it?
What do I gain for completing a whole chain?
Best Answer
By completing weapon side-missions, you gain Mirian (currency to upgrade stuff) and when you complete enough missions, the look of your weapons will change.
Some people mentioned, that you also get a few more execution animations and you break shields more easily, but that is only conjecture.
Sources:
Gosunoob.com - Sword legend
GameFAQs thread