I have encountered a couple of people who created them inadvertently. One young lady was dabbling with dark magic and at an age where she had wildly intense emotions, a lot of anger and frustration and it began to manifest itself as a shadowy entity. She basically created a tulpa or Watcher without any purpose and it scared her. With some guidance and a devoking and cleansing ritual she dissolved it.
I've personally concluded that a lot of the (undebunked) poltergeist activity in homes where there are teenagers are probably entities or tulpas born from combinations like the above: huge amounts of energy, very unfocused and wild emotions and dabbling in the occult.
Personally, I would stay far away of any such experimentation.