The Toxic Trait have been crappy in the previous iterations, but boy I think Only War has the worst one yet.
Believe it or not, but in one of the settings (Rogue Trader, I believe), Toxic actually does the same kind of damage as you did to the creature that had toxic. Yup. That means what you think it does. You just hit that beast with a Power Sword, so now you're going to take Energy Damage from it's toxins.
Just.. change it to melee only, or maybe a reasonable splash area, and apply some nice poison. The whole trait is just odd, no matter what book you pull it from.