Well of the Dark Side

The Well of the Dark Side was the source of the dark side energy on Mortis.

Ghosts of Mortis
Anakin Skywalker went there in pursuit of the Son to Obi-Wan Kenobi and Ahsoka Tano's dismay. Once Anakin made it there to confront him, the Son showed Anakin his future, including Skywalker killing Padmé Amidala, turning into Darth Vader, destorying Alderaan, and many other future events. Anakin was horrified of what he will become and the Son offered to join forces to destroy the emperor he saw in the visions and stop all the other events. Anakin hesitated at first but was seduced with the promise that their would be peace. Subsequently, the two joined together.

Soon after, Obi-Wan Kenobi went there to get Anakin back to their ship, since Ahsoka had finally finished the shuttle. Once he had made it there, Anakin confronted Kenobi, and the Son electricuted him, and stated that Skywalker was working with him, and both of them left the Well, leaving Kenobi. He then contacted Ahsoka and told her to disable the ship's engines, to her dismay, since she had just finished fixing it. He then added that it was the only way they could stop Anakin and the Son, but Anakin arrived just after. Luckily, she disabled the ship and escaped.

Just after her brief confrontation, Ahsoka got Kenobi on her speeder, and they left the Well. But before they did, she showed him the piece needed to run the ship, and added that their ship wouldn't go anywhere without it. They then left the Well hoping that they could stop the Son and Anakin.