Ice strykewyrms are a good source of blue charms, various herbs, crushed nests and pure essence. Also they drop the medium valuable staff of light. As with all strykewyrms, you must have them as a task in order to kill them (with the exception of one for a Fremennik Elite Diary task). They may only be assigned by Duradel/Lapalok, Kuradal and Morvran.