Niffins are powerful beings that are entirely magical energy that are created when a Magician fails to maintain an emotionless mind while controlling a spell and becomes consumed by it. The Magician is then enveloped by a blue fire that takes over their body, burning them into nonexistence. Once a magician becomes a niffin, their skin partly glows/burns with blue energy running through like blood in veins.
It is a general rule by most magicians that niffins must be "boxed" on sight, as niffins have been noted to be incredibly dangerous. Most niffins can be trapped in specially made boxes, but certain niffins have been able to escape them.
Products of a failed spell-casting in which either a magician continued to cast a spell but didn not have enough power to actually complete the spell in the first place or a magician unable to maintain an emotionless mind while casting a spell. Niffins are largely an unknown and less studied entities due to various reason, what is known is limited.
As they aren't human any longer, most niffin are rather violent and deadly. However, not all niffins have been attributed with this (e.g Alice), one insists that she doesn't feel the urge to kill anyone, but rather, she wants to be free to do whatever she wants and perform beautiful magic. Niffins can be contained like much like keiko demons, within a magician's body like a cage, but over time, the magician's body won't be able to hold a niffin for much longer and will eventually kill the holder. The power of a niffin has been compared to one of a god: Alice specifically allowed herself to become a niffin in order to kill the Beast and later express the capability to kill Reynard. Reynard claims that although alice, a niffin, is interesting but claims she would not be able to kill him. It's been claimed by a Niffin that they are able to perceive magic to a deeper and more profound degree than a Magician, and are able to perform more complex magic that does not require hand gestures. 
Those turned into a niffin display a vastly increased capability and ability for magic and all that it entails. The degree of increase is vast enough that a complicated unlocking spell that would've required a genius magician 4 hours, was solved in 15 seconds. Though this may be variable per magician who have been turned.
Alice once also sought out to reverse the niffin transformation and restore her brother to human. Quentin would later attempt the same with Alice and bring her back, but ultimately failed and set her free.
"Boxing" a Niffin
- ↑ The Magicians (Television): Season 2, Episode 9