Sometime during his training as a Padawan, a duel took place in the Jedi Council Chamber when Anakin Skywalker was confronted by a dark side entity in a powerful vision.
Sometime after the Battle of Naboo, during a lesson with Jedi Master Tera Sinube in the Jedi Temple on Coruscant, the young Padawan Anakin Skywalker sensed being choked by something he could not see.
The dark side entity blasts Anakin Skywalker with Force lightning
That night, feeling a disturbance in the Force, Skywalker left his dormitory and went to the Jedi Council Chamber, where a dark side entity appeared in front of him. Activating his lightsaber, Skywalker engaged the entity in a lightsaber duel and inquired it on its motives. The entity then told him that he would turn to the dark side of the Force.
Foreshadowing his future attack on the Jedi Temple, a group of defenseless younglings materialized in the Jedi Council Chamber. The entity told Skywalker that he could not save them, and that their fate was already sealed. The entity then subjected him to Force lightning. As he was losing all hope, Skywalker intuitively used telepathy to call out to Padmé Amidala, who heard his desperate plea far across the galaxy, on the planet Naboo. Focusing on his close friend, Skywalker filled up with energy of the light side of the Force and destroyed the entity.
Following the incident, Jedi Knight Obi-Wan Kenobi, warned by Tiplee, found his apprentice in the Jedi Council Chamber. The exhausted Padawan was then taken to his room, where he fell asleep. In the corridor, Grand Master Yoda explained to Kenobi that Skywalker had experienced a powerful vision and that perhaps, in time, they would understand it.