Question: can you get a legendary out of a lightning chest?
Answer: Mega Lightning Chests are not part of the regular chest cycle (see Chest Cycle). Mega Lightning Chests have a high chance of containing a Legendary given the chest is from P.E.K.K.A.'s Playhouse or above, and even guarantee one from Spooky Town or above. Mega Lightning Chests take 24 hours or 144 gems to unlock.