I think you get it by giving your vitality to some god. But this boost thing is actually kinda bugged, because it "disappears" when you use respec mirror, and makes you unable to get full hp: the max hp you can have is calculated considering this "boost", but the max hp of a character on the icon shows if there were no -2 const, so it kinda messes with hothead(yeah, i can explain i swear). So it's like you have 4370\4560 and you cannot get the actual full hp.
