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Is the lightning attack he used to destroy the Rebel fleet in ROS the most powerful ability he, or any other Sith in the mythos has ever used (aside from Force storms obviously)??

I think so

Just to clarify: Palpatine's lightning did not 'destroy' the ships. It caused the systems in the ships to crash, which essentially stalled them out and sent them plummeting to the ground. Once he stopped the lightning barrage, however, many of the ships instantly went back online and continued their assault.

That being said, I would say it was certainly one of the most impressive displays of raw power output from any Sith in the mythos, yes... Especially given the fine-tuned degree of control he displayed there(ie. each bolt of lightning *only* targeted a Rebel vessel.)

BvVT!

Starkiller did bring down an ISD in Legends, didn't he?

Originally posted by Eli Vanto
Is the lightning attack he used to destroy the Rebel fleet in ROS the most powerful ability he, or any other Sith in the mythos has ever used (aside from Force storms obviously)??

No. Darth Nihilus has used the Force to kill all life on a planet in a second - and he could casually TK a fleet of warships.

Makes other Sith like Palpatine, Tulak Horde and Valkorion/Vitiate look like weaklings by comparison.

Originally posted by StiltmanFTW
Starkiller did bring down an ISD in Legends, didn't he?

He redirected it as it was already falling. I mean it's still impressive regardless, but he didn't tear it out of the sky.

Didn't canon Luke bring down Star Destroyers with the Force in the battle of Jakku? That might be on par with what Sidious did.

We talkin' Legends or Canon? 'Cause in Legends, yeah, we've seen far more impressive things than simply bringing down a fleet of ships to the ground.

Force Storms allow one to do crazy things, yes, but it's not the ability that's powerful, it's the user. Palpatine and Vitiate could probably bring down fleets without force storms, as demonstrated by the Lusankya feat, the time he sucked the life out of everyone on Byss, used Battle Meditation on the entire Imperial fleet, etc.

And destructive output is not the only way you can measure power. The World Between Worlds is probably the most powerful force ability in both Legends and Canon, far more so than Palpatine's force storm/force lightning in RoS. And there's all sorts of junk in between.

Originally posted by Eli Vanto
Is the lightning attack he used to destroy the Rebel fleet in ROS the most powerful ability he, or any other Sith in the mythos has ever used (aside from Force storms obviously)??
Valkorion completely halted airborne shipping/smuggling activity over ASYLUM while confronting Arcann.

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Stray emissions of Valkorion's Force Storm ruined internal systems of military-grade ships used by smugglers (small and big) as well as electrocuted the crew. The ships began to crash into each other and worse.

Originally posted by Eli Vanto
Is the lightning attack he used to destroy the Rebel fleet in ROS the most powerful ability he, or any other Sith in the mythos has ever used (aside from Force storms obviously)??
Some of the most powerful applications of the Force are following:-

Valkorion's devastation of Ziost.

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Darth Nihilus's devastation of Katarr.

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Originally posted by Psychotron
Didn't canon Luke bring down Star Destroyers with the Force in the battle of Jakku? There are very strong implications that he did(and had the power to do so), but no explicit/incontrovertible proof as of now.