Vekeshi Blade

Composed of fire that has been kindled for five centuries since the death of Srasama, this weapon is pledged to defeat the infernal tieflings. You gain a +2 magical bonus to _attack and damage rolls_ made with this longsword. While wielding this longsword, you have _resistance_ to fire. On a successful hit against a celestial, elemental, fey, or fiend, this weapon deals an extra 2d6 damage. In addition, you can use an action to reshape this weapon into any other melee weapon that does not have the heavy property.

Source: Level Up Advanced 5e · OGL-1.0a · 5.1