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 Runetracer is the following:
NO FUTURE – Runetracer + Armor
Runetracer is unlocked by surviving 5 minutes as Pasqualina. Armor is unlocked by default.
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 ***
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
Unholy Vespers – King BIble + Spellbinder
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
Valkyrie Turner – Shadow Pinion + Wings