A Nemesis is an advanced type of Shadow Creature created by Tinkerbell with a specific person and their most exploitable weakness in mind. As such, a Nemesis' sole purpose is to exploit the weakness of the person they were created to destroy. Acting as either the embodiment of the person's internal struggles and insecurities or the physical opposite to their powers and skills, a Nemesis gleefully seeks to bring about the destruction of their "light counterpart": a desire that is seemingly instilled into a Nemesis starting from their conception and carried out after birth by their creator, Tinkerbell. Even if a Nemesis' loyalties do not lie with Tinkerbell like in the case of Vertigo, they will still seek out their "light counterpart" to destroy them with no qualms.
Their powers vary, ranging from void creation to petrification, but are dependent on the materials used to create them, as well as the weakness they were created to exploit. Additionally, unlike Tinkerbell's Shadow Creatures, a Nemesis is capable of free thought.
World of Winx
The first Nemesis seen in the series is Vertigo at the end of the episode "Neverland," after being created through the use of a stone pendant, an incantation and a bit of Bloom's essence stored within a Shadow Monster.