A 50k stone box is an item in Evony TKR (The King's Return). It can be found after killing certain monsters or bosses, with some of them listed below.
The description of 50K Stone Box in Evony TKR says:
Use it to gain 50,000 Stone.
50K Stone Box is known to drop from a number of bosses or monsters upon killing, including Pumpkin Monster.
Note: This list may be incomplete.
50K Stone Box is known to potentially be obtainable from opening Senior Resources Chest, Senior Alliance Boss Package, Lucky Box Ⅱ, Junior Lava Turtle Chest, Junior Golem Chest, Junior Witch Chest, Junior Warlord Chest and Common Ymir Chest. The links for each item are below:
Note: This list may be incomplete.