Biologist's scalpel. +3 to Attack, inflicts torture (after the hero with this artifact defeats an enemy hero, the enemy's castles and heroes are unlocked for 2 days).
Here's my artifact:
Lumberjack's Axe: +1 attack, for each Ent/Wild Ent/Ancient Ent killed, 1 unit of wood is added.
And also:
Hat - Mud Volcano: +3 defense, each unit attacking your squad receives Slow.
And for fun:
"Hair Growth" Potion: All enemy stacks are affected by "Overgrown! Let them shave!" They lose 3 attack and defense, 4 speed, and 50% initiative for one turn.
Okay, here's another idea:
Admiral's Sabre: +5 attack, +2 speed, +20% initiative if the battle takes place at sea.