The Giant's Stairs

From the publisher:
Rising from the sea to a height of nearly one hundred feet, the Giant's Stairs lie several hundred yards out from the harbor. The massive collection of basalt pillars does indeed resemble a huge staircase when silhouetted against the eastern sky. At most times it is nothing more than a rocky island, but once every seven years on the seventh night of the seventh month two of the fifteen planes intersect here and at gate is opened that allows passage into another plane where dwells a hideous giant. If any have ventured through this gate, none have returned to tell of it...