Toggle on veil of shadow

It is clear as mud when it says it must be toggled to maintain. One time? Or how often? Does seem visually like it fades a little after a few seconds. Noob question I know but i would rather be sure now instead of learning it wrong from the start.

every time you start a new game you have to toggle it

or if an arcane hero removes it

You just need to hit it 1 time, and then it’ll be active and you’ll see an icon for it appear above your health bar. If you die, you’ll need to reactivate it. There are hero monsters of the type “Arcane” which have a debuff to turn of your character’s auras, so that would be one of the rare times you’d need to press the button again.