This movie is by 2008, and the latest Indiana Jones' movie that I saw it yesterday night as 2nd time.
Well, this crystal skull is a sort of Alien(long-headed backward) that is waiting to be united with his/her crystal body as unified unit with other 11 crystal humanoids(!?).
According to my impression, there is just 1 multi-dimensional being who has anchored him/herself to this dimension, and in this desending process, his multi-dimensionality is divided into 12 crystals(or 13?). And when it reunite these 12 crystal bodies into one by hyper-circulation(as in Merkaba), it can return to the Being beyond time and space.
Well, Indeed Indy, the last scene is very intriuging.