Born Irek Ismaren but later to take on the role of "Lord Nyax," a Corellian boogey-man legend, and long rumoured to be Emperor Palpatine's son (he wasn't), he had similar Force-talents as Anakin Skywalker and became a strong Dark Jedi. However, in training with his master, he let his anger overpower him and his Master was forced to stab him through the head with his lightsabre.
However, despite severe brain damage, he managed to survive and had the added "benefit" of removing virtually all of his memory and mental faculties except for his ability to draw on the Force, which was also greatly enhanced, and monomanical hunger for more power, which he got by accessing the Force Nexus belong the old Jedi Temple on Coruscant (after which he could fly). To achieve this, he demonstrated the ability to project his monomania onto a large group of people so completely that they did nothing else other than want he told them - some would die from exhaustion and lack of food or water rather than stop working.
But as you can see, not only was he a monomanical psychopath with Force powers greater than virtually all, while recouperating from his stabbing, his mother had overseen his conversion from a Dark Jedi into a genetically enhanced killing machine "possessed with immense physical strength, having been medically altered to grow well past a normal human size [and] 8 lightsabers (whcih could not hurt him]... surgically implanted into his body along with sub-dermal armor pads."
It took three Jedi - Grand Master Luke Skywalker, Master Mara Jade Skywalker and Tahiri Veila - a Yuuzahn Vong army and a fleet to bring him down, and even then it is not confirmed that he is dead.