The chance is 5%, non-comulative. Each try is low chance + Korean random.
In my case I've made 2 cloaks legendary, both Elmore.
First it took me 11 scrolls and the second one (more recent like 2 months ago or so) took 21 scrolls.
There used to be 50/50 Legendary scroll, but it was kind of expensive and it still could have fall. You still have the option to use it, but for some reason the exchange option for it is gone, so these are practically non-existent at the moment.