Tedrasien is a tiefling artifact that was being documented and studied upon by the Order of Solis, but is now in Skamos' possession. It is believed to have been wielded by Akmenos, one of the greatest lieutenants of Emperor Bael Turath. The blade of this very wickedly-shaped sword will transform into a tongue of flame when the wielder has evil thoughts, the hilt remains unchanged. Virtuous thoughts will return the blade to its original, physical form.

It is a +1 flaming longsword (see page 234 in the PHB1)

As a free action, all damage dealt by the weapon is fire damage, another free action returns the damage to normal.
It has a daily power, where as a free action after hitting with the weapon the wielder can deal an extra 1d6 fire damage and the target takes ongoing 5 fire damage (save ends).