Well, I'll answer my own question. Modwyr grants the immunity to Intellect afflictions once you complete her soulbond. She also seems like an excellent, endgame-worthy sword. Legendary (for 30k gold), lash, stacking attack speed buff, immunity to intellect afflictions and gives a stunning per-encounter attack.
Unfortunately, the attack buff is only triggered by auto attacks not abilities as far as I can tell. Considering that most battles are over relatively quickly (playing on veteran), I never have more than a few stacks of the buff.