Caul or seventh child marks the cursed

Lobison

A man born the seventh son in an unbroken line of boys becomes one, and the change arrives on a Friday night whether or not he wants it: the sources are explicit that he can neither bring the transformation forward nor prevent it from happening.

Strigoi

Folk explanations for why a corpse might become a strigoi vary but recur across tellings: dying unbaptized, being born with a caul or an extra membrane, being the seventh child of the same sex in a family, having been cursed, or simply being improperly buried or mourned.