A 500k food box is an item in Evony TKR (The King's Return).
The description of 500K Food Box in Evony TKR says:
Use it to gain 500,000 Food.
500K Food Box is known to potentially be obtainable from opening Senior Spring Festival Rewards, Medium Spring Festival Rewards, Lucky Box Ⅷ, 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.