Weak against Relict Oil.
Dopplers, also known as vexlings or changelings, are beings able to take on the form of any humanoid or animal they please. This transformation is no mere illusion able to be shattered with a simple spell, but an authentic and complete metamorphosis. This means no protective amulets or witcher medallions will signal a doppler's presence - they emanate the same aura as the being whose form they have assumed. This, combined with their incredible intellect and cunning, would make dopplers supreme assassins or thieves - had not nature endowed them with generous and timid spirits which make them avoid the shedding of blood at all costs.