I was about to ask the same question.
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Spectral Terror has two powers to it.
One is a toggle that effects those in melee range of the Spectral Terror, every 10 seconds here.
The other is a click that is a ranged cone, every time it fires it there will be a chance with that.
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I did some testing on the trolls in Skyway, Natsuki is correct, it fires on summon//toggle, and when ST uses its cone. Effective, and scaring people to death is good times.
Thanks for the excellent guide Natsuki!