Evolving weapons happen by combining a max level weapon with any level of a specific secondary support item. Once you have both of those items you must then kill an Elite enemy and open the chest that it drops. The combinations for the Shadow Pinion is the following:
Valkyrie Turner – Shadow Pinion + Wings
Shadow Pinion is unlocked by surviving 15 minutes with Concetta, a character who must first get unlocked by opening the Vampire Coffin in Gallo Tower. Wings are unlocked by reaching Level 5 as any character.
If you're interested in seeing what the combo is for the other weapons they are listed below in the spoiler tag :)
*** Spoiler - click to reveal ***
NO FUTURE – Runetracer + Armor
Unholy Vespers – King Bible + Spellbinder
Bloody Tear – Whip + Hollow Heart
Hellfire – Fire Wand + Spinach
Death Spiral – Axe + Candelabrador
Holy Wand – Magic Wand + Empty Tomb
Thousand Edge – Knife + Bracer
Heaven Sword – Cross + Clover
Soul Eater – Garlic + Pummarola
La Borra – Santa Water+ Attractorb
Thunder Loop – Lightning Ring + Duplicator
Gorgeous Moon – Pentagram + Crown
Vicious Hunger – Gatti Amari + Stone Mask
Mannajja – Song of Mana + Skull O'Maniac