Complex sentence: The most spectacular feature is the huge freestanding rock called "the Stack", at the northeastern corner of the island, which plays host to a large variety of seabirds, including thousands of guillemots, razorbills, fulmars and gulls.
Split into simpler sentences: The most spectacular feature is the huge freestanding rock called "the Stack", at the northeastern corner of the island. The island as a whole and especially the Stack, play host to a large variety of seabirds, including thousands of guillemots, razorbills, fulmars and gulls.