The following is based on Lionheart and is not considered canon.
“Thorny growths erupt from your body at odd angles and in uncomfortable places. Luckily, these thorns break off and grow back easily and give anyone that attacks you a face full of pain.”— Game manual, Lionheart: Legacy of the Crusader
10% of all physical damage given to you is given right back to your attacker. You also receive 10% bonus to your Crushing, Piercing and Slashing resistances. The down side is that your Charisma is reduced by 1 and your Acid, Cold, Disease, Electrical, Fire and Poison Resistances are all decreased by 10%. This trait, like all sylvant traits, will mark you as tainted by magic and will cause some people to react negatively to you.