In your case, Lucario- EX is Basic, M Lucario- EX is Mega evolution that "Evolves from Lucario- EX " so you only need Lucario- EX in play to be able to evolve to M Lucario- EX , any other combination won't work [1]
This is black's take on a Clone effect, allowing you to pay 3 mana at flash speed to have it enter as a copy of any creature in a graveyard that died this turn
Yuriko, the Tigers Shadow is perhaps the least competitive of the four
Xyris, the Writhing Storm Magic players love drawing cards, so its always nice to see a bit of a reason to make this a downside
Akki Battle Squad Akki Battle Squad is a 6/6 for 6, which is already good
I dont like to use the word addictive when writing about games as I find that it too often trivializes more serious problems, but its fair to question how all of this is constructed to let players buy more dopamine hits every day