Registered: Aug 2014
Location: The balance of the Force
Malgus was only beaten by an entire strike team in a battle that has been described as 'intense', and he's repeatedly overpowering and blitzing the team in cutscenes. Sel-Makor states that Baras decades prior was greater than prime Fulminiss, in a discussion specifically about power. Baras gets far more powerful in that time.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
We only see a cutscene where Malgus catches them off-guard, and we know the protags were stated to have fought through armies and were tired by the time they made it to Malgus. We also don't know how well Malgus fared before going down. It being intense means jacksh!t, since Sidious vs Savage Opress was also described as "intense".
Sel-Makor is hardly an authority over Baras and Fulminiss, but at the same time what does Fulminiss have to his name? Here's the quote I assume you're referring to:
"Widely regarded as one of the greatest Sith sorcerers in the Empire, the enigmatic Lord Fulminiss is a master of the dark side’s most arcane and lethal aspects. The grotesque Harrower assassins are his most famous creation, but legend has it he once summoned a raging storm of pure Force energy that disintegrated a rebellious city of natives in the Imperial-conquered Jabiim system."
-- Lord Fulminiss Codex Entry
From this we know a "raging storm of pure Force energy" is a legend(i.e. I wouldn't place much stock into it) and he's also a Sith Sorceror who's known for his rituals. This could've very well been another ritual, assuming it even happened.
Registered: Aug 2014
Location: The balance of the Force
Except it's clearly a hard fight, pretty much every character says something along those lines.
Sel-Makor gives both of them boons of power and is an entity that thrives on feeding off of the psychic energy of other beings. He does the same with Vitiate's voice.
It's repeatedly referenced by numerous other NPCs, including Fulminiss himself. Though as Kbro has pointed out before, that's just a world-building device.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.