I have seen countless metal chests / master chests (yes, thats what theyre called ingame (highend)
This is how they work:
A master chest can drop from any mob in the gme. I only know of highend ones tho (40+) but its something like 1/100-1/200 on nameds, and 1/10000-1/20000 on normal mobs. This is the ~chances of them dropping.
A master chest can hold a 'Fabled' item or a master spell.
A master spell is a random (almost always) spell from the zone loot table.
However, a Fabled item is very nice, have about double # of stats on them, far greater damage (if weapons) and often have procs.
A fabled item is locked to a certain mob, if you get a fabled item, you should know that the mob you got it form is the ONLY mob in the game dropping that item.
I have been hunting metal chests for 10 lvls (40-50) and ever since i hit 50. I thikn i have seen 80-100 of them. Winning maybe 15. Using what i can use, selling the rest. Standard price of 40+ master spells on my server is 2pp, if you get a good one its more like 5-8pp. I myself have about 50pp to my name, and no, I do not harvest and I do not tradeskill at all. So its a good moneymaker, however i spent months to find the good places and things like that. Figure out patterns and spawn times etc. and youre on your way.
Hope this clarified it for some people.