Hidden quest with some key

Hello, i was playing and while that the player from another team called my atentiom to his base where was a panda, so he tell me that was a quest with the key in the left top corner just that....i alred tryed to figure it out lot of times but i just dont know how...i alred looked for some answers here and all i finded is to kill some shadow or ghost but ive never sawed him....can somebody help me with that?

This quest was never finished. There used to be 3 skulls you could get; and all 3 were supposed to turn into an item that would randomly change your heroes color. Lab never finished the quest; thus rendering it pointless. Since then 1 of the skulls has also been removed, and in 4.0 the entire quest is gone. (though I may be adding on a quest of my own!)