Basically I opened a Level 51-55 chest when I opened it I got a silver box with a A on the front of it. It says it's legendary, I think it upgrades your armor/weapon just need details.
It's used to improve a selected stats on one of your item, for exemple u have a +15 focus glove, use the silver box and select the 15focus stat, then it'll or success and increase this stats, or it'll just fail =X
You can use it on gear, basically what it does is you can select a stat from a piece of gear of your choice, and the box randomly selects a value.
For example: if I want con on my legendary level 58 TMQ boots, I right click the box, put the boots in, select con from the list and then the box randomly chooses a value of con from 1 - 29 con.