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How much did it destroy? Apparently it was going to destroy the surface of the entire planet.

On Ruusan? His only telekinetic showings on Ruusan was breaking a Sith Lords alone and breaking boxes and tents lol.

It was the Force Wave he told other Sith Lords to generate their energies unto him and destroy a forest.

That's not a Force Wave, though. More like massive-scale FLS ritual.

How much did it destroy? A forest.

Darth Bane learned the ritual from Darth Revan's holocron, by the way.

"... united the minds of the Sith through a single vessel so their power can be unleashed upon the physical world. The storm rolled down from the plateau and rumbled across the forest. Hundreds of forks of searing lightning shot down from the sky-and the forest erupted. Trees and spreading out in all directions. The underbrush smoldered, smoked, and ignited; and a wall of fire swept across the planet's surface. The inferno consumed everything in its path."
―Darth Bane (Darth Bane: Path of Destruction)

It was a ritual and if I recall it was a Force Storm calling lightning from the clouds and leveled an entire forest. And yes it was a ritual with a bunch of sigh channeling energy through him and Bane was levitating

It wasn't his own energy though, it's hardly applicable as a feat.