Shivers are Silvers who can manipulate ice. However, their ability is limited only to freezing and frost inducement.
It is shown in Red Queen they have the ability to freeze blood inside of bodies.
It is seen in Glass Sword that they are capable of turning moisture into ice spikes and throwing walls of them at a time.
"Shivers are elementals, manipulators of temperature and cold, able to freeze things dependent on properties of the target. Most shivers can easily manipulate water or more stable liquids into ice." -Victoria Aveyard, "War Storm" "On Silverblooded Abilities"