A strange humanoid figure enters the arena from the western gate. Other than being quite thin, it lacks any distinguishing features. As it draws near, its body appears to melt like wax against a flame. As its skin becomes pliable, the body begins to transform. - Tome of Encounters (RPA-SQ1) 

The Opaunka are a type of Shapeshifter.

A being that can assume the form of other creatures, including a Dragonkin, Halfling, Human, Ogre, Satyr, and Vampire.

The shapeshifter can generate weapons with which to fight when adopting the form of another Race;

The figure takes on the appearance of a Human warrior and charges forward into battle. It raises its sword high, and bright metal glinting in the light of red suns. - Tome of Encounters (RPA-SQ1)

They can also assume the likeness of any being its adversary has seen, able to inexplicably draw on the memories of its target;

To your surprise, the figure takes on the appearance of Jara, the Satyr leader you met in the Forgotten Forest near Falandore. Swinging her spear, she charges forward into battle. - Tome of Encounters (RPA-SQ1)

When killed, a shapeshifter will revert back to its featureless self;

When you kill the creature, it reverts to its original form. The strange, featureless humanoid dissolves into the ashes as the crowd cheers and chants your name. - Tome of Encounters (RPA-SQ1)

You are met by a curious shapeshifter in the Colosseum of Ashes. - Tome of Encounters (RPA-Tome-SQ1-171)

• You best your foe and the shapeshifter returns to its natural form as it turns to ash in defeat.
• Your foe bests you and takes on your appearance for a laugh.