This often represents the life of a person now dead, perhaps from the long past. It is also a common symbol of human death, mortality and frailty.
It can also depict the old views of life we have gained form our ancestors. See The Conjuring Trick
A lit up or displayed skull: An easy acquaintance with death, or the easy memory of the person whose skull it is
Digging up skull: Possibly you are meeting feelings of death or discovering past ancestral memories; or even finding memory of part of you murdered. To- find out you could use - Being the Person or Thing