Can anybody tell me what is a core MK II?
It’s equipment gear like anything else, but this is used for hero’s that are already platinum, to give them plat +1 etc
Ohh, I don’t even think I’ve seen a plat +1 hero yet. I bought a few of them thinking I would use them someday for something special in the game. (Not for plat heroes) I feel I wasted my $ now. I’m far from having any plat heroes. 
They don´t decay, so you can use them later 
It might be aggravating (is the the right word? Googled it, but there were so many translations, I hope you get what i want to say 
) now, but stuff like this happened to everybody. Just play on and later the price you paid for them will seem much less.