Seems to be about a 5-10% chance to get from a Brown Treasure Casket in Sauromugue Champaign. Getting them on the first one is just extreme luck. I myself took about 10-15 Caskets before I got mine, and a friend hasn't gotten his after 10-15+ Caskets.
took me a week of farming to get these. While a chest might have a 5-10% chance of having the subligar, (i think closer to 5%), keep in mind brown chests have a very low chance of popping in the first place, (i was getting close to 3 every 2 hours) and then the odds of the casket giving you the clues to actually solve the casket is a factor as well.
Highly recommend waiting until the kupower "Mystery Boxes: Windurst" is active, as it doubles your chances at finding a locked chest. Spent about 1-2 hours with that up, went 8/9 in unlocking chests and 1/9 in getting this. Good luck! ^^
Back when i farmed these, i remember farming for 22 hours and opening oer 100 brown chests and failing another 50-60 or so.
Typical, if you really want them, you dont get them. If you dont need it, you get it straight away. (killed 1 mob in Sanct and got a brown chest containing Rossignol, go figure)
As for the item. So far i dont notice much of a difference tbh.
Bst only at 64 still though, mybe it will get better at 75.