Psionics

Psionics is a scientific practice that isn't really well understood despite that it has been around since centuries ago.
By focusing mental energies, a user -called a Psionicist- can harness the power of the mind to alter wither their own mental state, or that of another.
As psionics developed, they became a lot more powerful. They could now directly alter states of matter and permanently damage people's minds when in the wrong hands. This lead to the practice of psionics being outlawed in most places, save from a small number of monasteries that were very well trusted.
Not long after this, it was discovered that psionic powers come at a cost. The slow decay of a psionicist's brain became evident after around a decade of master's study. This was discovered when one of the monasteries fell out of contact and, upon investigation, the psionicists inside were found, some catatonic, others with severe brain damage resulting in near helplessness.
After these events, knowledge of psionics fell to scholars and catalogers. Only few brave enough to dabble in the powers, but only minutely, lest they suffer a fate like the monks of old.
In the new age, psionics are mostly forgotten. Only very select centers of knowledge really have any idea what it actually is, and it is commonly referred to as 'old magic'.

Discovery

Humans discovered Psionics in 502AD. It was in a monastery of thought, where its people were convinced that the mind had to be more powerful than simply conscious thought.