Skulls and snakes are frequently seen in the Harry Potters series.
The series major villain, Voldemort keeps a pet snake and is able to communicate with them by using the snake’s language, Parseltongue.
Harry Potter is also fluent in Parseltongue thanks to the same childhood incident which gave him a lightning-shaped scar on his forehead.
Wizards in the Harry Potter Universe are separated into four different competing houses at school. The emblem for Slytherin (slithering, get it?), the house from which most villains from the series emit from, features a serpent.
Voldemort forms a villainous secret society known as the Death Eaters who used a snake and skull symbol, the Dark Mark, as their emblem. Members of the Death Eaters’ inner circle a Dark Mark burned into their forearms which can be only revealed with magic.
The Dark Mark is summoned after a Death Eater has murdered. The group is obsessed with ridding the world of non-wizards.
Dark Mark being summoned post-mass slaughter:
Image Source: Harry Potter film series, Warner Bros.