The Zamorak robe top is part of the Zamorak robes set. They are bright red and show allegiance to the god of chaos, Zamorak. They are dropped by several Zamorakian followers such as Disciples of Iban and Zamorak wizards.
A set of Zamorak robes is required during the Underground Pass quest, in which they must be worn to gain entrance to Iban's throne room in order to defeat him. After the quest, they are no longer required.
This robe top counts as a Zamorak item in the God Wars Dungeon, and when entering Castle Wars waiting rooms.