Irek Ismaren. Later assumed name of Lord Nyax.
As a child his brain was cybernetically enhanced allowing him to mind control droids. At that time he attempted to use Eye of Palpatine to dominate the galaxy by controlling mind of the ship. Later having his brain seriously damaged was put in stasis and after horrible experiments he became almost 3 meters tall, his Force power was enormously imbued by the implants in his head and he had 8 lightsabers attached directly to his body: 3 on each head and one from each knee.
He got free after Yuuzhan Vong occupied Corruscant. Unlike other Jedi, he didn't have trouble tearing Vong apart. His ability to control minds of others was so huge that it took combined effort from three powerful Jedi: Luke, Mara and Tahiri to resist him. Luke in terms of Force power was weak at that time and, if he was alone, there would be fair chance of him becoming his puppet. All three engaged him in lightsaber combat simultaneously and couldn't penetrate his multiple saber defense. A Yuuzhan Vong admired his strength and, yet, saw that his moves did not have sequences, that with training he would become even stronger.
Moreover, he tapped into Force Nexus and after was able to levitate and surround himself by Force storm, while Luke and his companions could use same Nexus just to throw a few more stones around.
Basically, he was described as unstoppable. Had there been other circumstances, who knows what threat he could pose to the galaxy.
The single fact that Luke was brought to him by Force visions already proves that it was a serious threat that rippled the Force with alarm.