Arcane Cloak Mechanics?

Wondering exactly how Arcane Cloak functions in terms of the lightning damage gained from mana spent

Specifically, is it increased flat damage to just attacks, just spells, both attacks and spells or is it all skills?

Main curiosity is if it applies to Herald of Thunder's storm (I am aware that if it's damage to attack or spell it wouldn't work)

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Buff grants Added Lightning Damage equal to (10–14)% of Mana Spent by this Skill's effect

Not specified, that means Arcane Cloak give flat damage bonus to all hits.
Including seconadary damage from HoT.