I discovered this bug last night when I learned this rune. There is actually two bugs.
1) 85 damage. It displays that it does 85 damage in the recipe, in your bags, when you learn the recipe, when you make the rune... but when I applied it, it was changed to 88 damage. Nothing major, just a small little bug I found.
2) Dual-Wielding. This was the major bug. The Baneful Rune states that it is applied to "Main Hand" only. After applying it to a weapon, I was able to put that weapon in my offhand. Out of sheer curiousity, I contacted a GM and made a ticket, wondering if it was possible to essentially have two weapons with Baneful Rune both procing. As the rune was changed recently in a patch to do all the damage up-front (as opposed to damage over 4 seconds), the fact that the two runes would share a proc would be meaningless as both would deal full damage immediately.
Its 85 damage though. I'm unaware as to the exact chance it has to proc (no information on ZAM wiki). So I don't know if this would simply be a fun stunt, or if it would actually be worthwhile to do so.