I assume that skill is only 3 min is PVE but inside PVP maybe it only slows down or only last a few seconds.
Three minutes is broken as hell for that ability even in PVE, I don't care what the cooldown and what not is. Even if all the damage and mana use is immediately applied when the effect ends, not really a fair trade off. He couldn't take damage or run out of mana for
three minutes. You could unload an absolute torrent of abilities in that time. Honestly, I'd rather assume this was a mistranslation or something. Maybe even a typo in the raws. You might have to rebalance an entire game around an ability like that if it actually lasts that long, which would just make everything else weird as hell.