Casting Shadows is an achievement that requires the player to kill a shadow in the Temple of Light with a shadow spell. Earth runes and either death runes or blood runes, as well as a magic weapon are required to complete this achievement.
Shadow spells are one of five types of Ancient Magicks combat spells. An ability will not work; the runes must be spent to cast the spell and kill the shadow with a regular auto-attack to get credit for the achievement.
The shadows in Prifddinas do not work for this achievement.