Whitebeam, commonly refers to the Sorbus aria, a small to medium-sized deciduous tree common in Great Britain.
The tree, related to the Rowan tree, is a cousin of sorts to Mountain Ash.
Within the mythology of Hymn For The Cursed, Whitebeam is only ever seen used by Werewolves. It blocks and exhibits control over humans and other natural/unnatural objects or beings, including bullets. While a common plant, its abilities are know to few, and its secrets to unlocking those abilities are known to even fewer.