A 500k 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 500K Stone Box in Evony TKR says:
Use it to gain 500,000 Stone.
500K Stone Box is known to drop from a number of bosses or monsters upon killing, including Pumpkin Monster, Cranky Snowman, Senior Nian, Medium Nian, Junior Nian and Ifrit.
Note: This list may be incomplete.
500K Stone Box is known to potentially be obtainable from opening Senior Spring Festival Rewards, Medium Spring Festival Rewards, Lucky Box Ⅸ, Lv13 Boss Monster Chest, Senior Witch Chest, Senior Warlord Chest, Lv15 Boss Monster Chest and Grand Ymir Chest. The links for each item are below:
Note: This list may be incomplete.