Dropped from various monsters on corresponding day and weather. Weather has a higher drop rate than day tho.
Basicly anything that gives xp under weather effects or day of the week has a chance of droping these. They are used to upgrade all thunder based Elemental path weapons from trail of the magains. This means added effect of Lowers monster Acc and boosting player dex and Acc.
Prices at the min are sitting at 10-20k and you need 15 per weapon, expect the market to flood and the price to fall as more becomes freely known about how to get these.
