The Assault on Mount Tantiss, alternatively referred to as the battle of Mount Tantiss, was an operation executed by operatives of the New Republic. The aim was to neutralize Grand Admiral Thrawn's cloning center situated within Mount Tantiss on the planet of Wayland.
During the events of the Thrawn campaign, Mara Jade, formerly an Emperor's Hand, was apprehended by the New Republic. The reason for her arrest was a suspected connection to an Imperial raid targeting Coruscant. She later disclosed to Luke Skywalker her knowledge of the whereabouts of Thrawn's cloning facility. Skywalker then made an agreement to free her in return for the coordinates leading to Wayland.
In a parallel development, Imperial Colonel Kleyn Selid discovered that Joruus C'baoth was responsible for the death of General Freja Covell through mental manipulation. Selid reported this to Grand Admiral Thrawn, who then gave the order for C'baoth's arrest and confinement within the Emperor's throne room. However, the Dark Jedi managed to dispatch Covell's troops to eliminate the ysalamiri that Thrawn had strategically placed near Mount Tantiss to keep C'baoth in check.
On Coruscant, Leia Organa Solo had a meeting with the smuggler Talon Karrde. He informed her that the Empire had taken a considerable number of ysalamiri from Myrkr, suggesting that it could be a setup intended for Skywalker. Consequently, they decided to travel to Wayland together to deliver a warning to their comrades.
Jedi Knight Luke Skywalker, accompanied by Mara Jade, Han Solo, Chewbacca, and Lando Calrissian, made planetfall on Wayland. From there, they journeyed through the planet's forests, heading towards Mount Tantiss.
Their initial encounter was with a contingent of Noghri Death Commandos who had arrived on Wayland to support the son of Darth Vader. Subsequently, they encountered some of the planet's inhabitants, specifically the Myneyrshi and the Psadans, and successfully persuaded them to maintain neutrality. Nevertheless, some Myneyrshi ultimately chose to assist them in their fight against the Empire.
They eventually gained entry to the Mount Tantiss facility, engaging in combat with stormtroopers. Calrissian and Chewbacca branched off to locate a means of destroying the facility, while Skywalker and Jade set out to find the facility's remote self-destruct mechanism. Within the cloning chambers, Calrissian ultimately managed to set up an arhythmic resonance scheme designed to obliterate Mount Tantiss. Meanwhile, Skywalker's sister, Leia Organa Solo, entered the facility alongside Talon Karrde.
Skywalker and Jade made their way into the throne room, where they confronted the deranged Jedi clone, Joruus C'baoth. C'baoth eliminated the ysalamiri that suppressed the Force, and then revealed Luuke Skywalker, a clone of Skywalker. Skywalker and his clone faced off, while C'baoth used a burst of Force lightning to incapacitate Jade.
Eventually, Leia Organa Solo, Han Solo, and Talon Karrde entered the throne room. C'baoth unleashed a Force scream, which stunned everyone present. Capitalizing on the Jedi Master's distraction, Jade managed to use the Force to retrieve Organa Solo's lightsaber, and then killed Skywalker's clone, thereby fulfilling the Emperor's final directive. Enraged by the death of Skywalker's clone, the Jedi Master began to collapse the ceiling upon Jade and Skywalker.
However, Jade utilized Organa Solo's lightsaber to create openings in the floor, causing rocks to fall through and move against C'baoth. Guided mentally by Organa Solo, she reached the insane Jedi and struck him down with the lightsaber. He then exploded in a blast of dark side energy.
The destruction of the Mount Tantiss facility brought the Imperial cloning operation to a definitive end. Coupled with Thrawn's death at the Battle of Bilbringi, the day's events were a major setback for the Empire and effectively concluded the Thrawn campaign.
Skywalker's duel with his clone had a traumatizing effect on him in the days that followed.
Following the destruction of Mount Tantiss, Wayland became a planet of significant interest to the New Republic. Researchers and Intelligence officers thoroughly examined the planet and its ruins, searching for clues to aid in their efforts to defeat Imperial warlords. This led to the discovery of evidence that would eventually result in the political repercussions of Bothan involvement in the destruction of Caamas. The New Republic also relocated the Noghri to the colony of New Nystao on Wayland, allowing them to escape their collapsing home world of Honoghr.