A Steelthorn is a level 75 weapon that is break on death. Although any path may use it, it is said to have special bonuses for rogues.
Durability - 500000/500000 Damage - S: 80m95 L: 70m90 AC : 0 Hit : 3 Dam : 5 Vita : 0 Mana : 0 Might : 0 Will : 0 Grace : 0 Protection : 0 Healing : 0 Peasant Level 75 Might to Wield : 0
You may obtain a steelthorn by winning one in a legends or higher riches carnage or as drops from muck/slime ogres or from dirt/shadow mantis.
This price for this item varies and you may check http://www.merchantcircle.net (Under the Trove, services, price list) for the current market price.