When you are fighting Kael'tas in Tempest keep in phase 2 he spawns weapons and after they are killed you can loot legendary weapons. Weapons will deappear after whipe or leaving tempest keep.
When you see your raid will whipe and you have weapon go to safe spot and press alt+f4, if you stay alive 20sec while keal is engaged you will keep weapon when you log in.
We noticed too staff buff will be in all groups if 1 ppl have staff equipped in next try.