Dark Portal is a special skill which is only usable on Destruction difficulty after killing certain super unique monsters and opens a portal to a certain uberlevel when used, depending on the monster that got killed. This skill is unlocked by transmuting the Class Charm with the Dark Tome dropped by The Summoner on Destruction difficulty.
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Dark Portal can also be used after interacting with the figures at the end of the Diamond Gates and the Desolate Road to open portals to the Edyrem's Path and the Nexus Bridge respectively.