Jack Valentine's Bio

When Jack heard of his brother's sudden admission to a mental hospital, he rushed to see what the matter was. As soon as he got there, his brother warned him of the notebooks and the beings beyond the "veil". not long after, it was time to clean his brother's apartment, jack finds a 2 strange books and an arrowhead, after cutting himself with the arrowhead, strange things begin to happen. What's with these strange ghosts, what is an eldritch being, and why are they thirsty for jack.
(no art is mine and any media used belongs to their rightful owner)