Treffilli is a herb.
A sprig of this plant makes a spell which can make you invisible or others sleep. It should only be used when one feels it vibrate. To make a foe sleep, one sprig should be crushed and then the following words should be whispered "By Ash and Bird".
To make oneself invisible crush two sprigs and the following should be whispered "By Ash and Bird and Crust". But the power of the spell is such that each time a sprig is used in these ways the stamina of the spell caster is drained.