Anywhere OS stuff drops these can happen.
I tried really hard to figure out a usable build with these (w/ zealous mod for -2 regen). I tried an adrenaline based hammer build, it kinda worked but pretty much any skill over 5e was out most of the time. I tried a warriors endurance axe build figuring the energy gain from WE would counteract the energy degen, it kinda worked, Having the sunspear whirlwind attack adrenaline skill in mobs for free energy gain was about all that made it usable.
I haven't tried a ranger with dual zeal bow but at least they will still have 1 regen, warrior gets nothing lol.