Same everyone else said.... Leakdown check, borescope...
When checking the rockers/pushrods, try wiggling the top of the #6 springs- looking for a hammered valve guide. It sure looks like oil consumtion in that cylinder, to me. A ton of carbon there compared to the others.... That could lead to a hot spot, then detonation, blowing off that electrode. (Remote possiblity....)
Does it smoke out the exhaust at all? Pull the pcv- smoke out of the valve cover? Out the dipstick tube? Does the motor have any new/different sounds or noises?