[quote=@Ithradine]I think I will just go with sharing the same XP bar[/quote]
You could treat it the same way as I am with Arwyn's Estent Magoli where their power is based purely on her Strength. That way, if you invest XP in increasing Arton's Strength, the blade would also get stronger.

By the same logic, any XP spent buying new swordsmanship abilities could be written in a way to imply that it's the blade which is getting stronger, rather than Arton. This again would use the same pool of XP as everyone else, but since the abilities would be tied to Severance you could tag them as requiring it for them to work, similar to what I did with [i]Arcane Arrows[/i]/Gúl Peng Arwyn and [i]Shadowmeld[/i]/Gwathren Gollo.

What would the damage reduction of Oathkeeper be?