Griswold's Treasure box can be found by killing Griswold in Tristram on Hatred difficulty. You may do so as many times as you want.
Cube with Oil of Craft to open box
As the description says, you can cube it to get some items.
Griswold's Treasure + Oil of Craft → 3 Tier Unique items (based on Character Class)
It always gives the same predefined unique item classes depending on your Character Class. It might give you something other than what's listed below but the chances of it are very low.
Items can be Tier 1 or 2.
||Items from Griswold's Treasure
||Barbarian Class Item
||Druid Weapon or other weapon
||Sorceress Weapon or Staff
Hover over an underdotted word to see Full list of what items are included in that item type.
NOTE: These Uniques cannot be disenchanted, but can be disenchanted normally after uptiering them.