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This article contains plot details from one or more of the books.
Obea Trees receive their name because only one of it's thousands sprouts grows into a tree, and that takes hundreds of years, a bit like the barren Obea wolves. They also have another name, Witch's Tree, for their black spectral branches which claw at the night sky. They also have buttress roots, which are roots like walls in a cave, that flare up to support the tree because the ground is to hard for the roots to dig in. The roots are a great place to hide things.
HistoryThe Sark and Gwynneth are looking for Gwyndor's mask, which has been hidden by a wolf known as the Prophet. They approach an Obea tree, and believe it is the perfect place to hide something, as it is rumored standing near such a tree can cause you to become barren. The Sark finds some food and quarrel about Gwyndor's death.
They begin to eat, and sleep near the tree until the Prophet arrives.