Rosslyn Chapel is actually called the collegiate chapel of St. Matthew and it was built in the town of Rosslyn, Scotland and was commissioned for construction by the Earl of Orkney in the mid 1400's.
The myth surrounding the chapel is the historical rumor that the Templars had hid there a piece of the cross where Jesus was crucified, his embalmed head, and the Holy Grail, among other things. Although this is rumor, additional assumptions have been made regarding the construction and religious symbols that are part of the church.
For example, according to the main character, the walls of the church display Masonic, Jewish, Christian and even Muslim symbols. Rosslyn's carved woods display anything from a camel, to dragons, to deities known as "Green Men, and it is all displayed in a chaotic and cluttered manner.
The chapel is also thought to have been built by Freemasons, which is not true, as it is also untrue that there are Masonic symbols on the church. The main reasons why Rosslyn chapel has been made the center of so many controversial legends boils down to two very important factors: money and lineage. This chapel was commissioned by one of the most rich and powerful titled Scottish families and every whim of their imagination was brought into the design of their own chapel. That "legends" and fake stories are given to validate the eccentric and Gothic design of the place was an expected practice among well-to-do medieval families.