Currently, the Poison Tomahawk does not seem to do anything that the other tomahawks can do better.
Most, if not all creatures that are weak against affliction magic that wouldn't be dispatched in the same or less amount of hits by a fire, lightning, ice, or even dark tomahawk are either constructs or undead, which are immune to poison anyway. (So they don't take any damage from the tomahawk)
Swapping the damage from poison to acid would make it much more useful.
Most, if not all creatures that are weak against affliction magic that wouldn't be dispatched in the same or less amount of hits by a fire, lightning, ice, or even dark tomahawk are either constructs or undead, which are immune to poison anyway. (So they don't take any damage from the tomahawk)
Swapping the damage from poison to acid would make it much more useful.