Complex sentence: Games Workshop has not acquired the rights to "The Silmarillion", which is still the exclusive property of the Tolkien Estate, but has the right to develop its own derivative intellectal property to fill in the gaps in The Lord of the Rings Universe.
Split into simpler sentences: Games Workshop has not acquired the rights to "The Silmarillion", which is currently the exclusive property of the Tolkien Estate. However, the company is allowed to develop its own derivative intellectal property to fill in the gaps in The Lord of the Rings Universe.