@ Liltrouble (and everyone else going wtf @ drk on this) DRK is on a lot of PLD shields. It doesn't hurt the item at all, and I doubt SE intends for DRK to shield-tank. Here's why it's on these: It's a nod to FF4. Cecil starts out as a DRK and, being told he can't fight evil while evil, quests to become a PLD.
I confirm its from Brown Caskets in Fei'Yin,
Took about 37 hours to attain it. In the 37 hours I got 15 brown chests and thats with full mog tablet set + 3 Prowess: Increased Treasure Caskets, from doing FoV pages
Found in a Brown Casket in Fei'Yin after 17 cumulative hours of active farming, 9 of which were devoted to building Casket Prowess Lvl5. Can also confirm that it can load in a chest that was dropped by a Too Weak mob.
Besides, what I don't think people understand is that DRK didn't bump out another job for this. If DRK weren't on it, it would be PLD only, it's not like your RDM/WHM/w.e you presume this would be useful on, got screwed.
Another shield goes to DRK. DRK has no native cure abilities and no shield skill. With the update to last resort there is very little reason not to use a 2-handed weapon on DRK.
edit: @ Alistaire its not that drk on shield is a problem, the problem is which jobs are not on the shield ie RDM and WHM could get some good use of this and have shield skill