The item Throwing Star of Sula was implemented on Winter Update 2015 with the objects Behemoth Taming Stone and Demon Oak Wood. The three of them were never mentioned on any official source of Tibian lore, and they can only be seen in a single place which the devs refused to give an official name and is known as the Schrödinger's Island due to its unconfirmed existence's history.
Given the fact that this island can only be reached by characters at level 999 or above, and that it depicts a chilling place where players can relax and swin at the beach, or buy food, drinks and even a drum from NPC Relaxed Bartender, it looks like the Island was designed to be a secluded place where high level characters can rest away from low level players and take a break from their countless hours of hunting. Therefore, any exclusive-items and objects that can be seen on it are there just for role play reasons or as red herrings. Having said this, we can conclude there's no story behind these items and, most likely, there will never be one.
Although each of these three unique objects that were implemented together have names that mention other game elements, it doesn't necessarily mean they are related to Sula's, Behemoth's and Demon Oak's own lores. However, one interested in knowning more about Sula can read about her on the Tibian Genesis - Chapter III: The Birth of the Elements.
Curiosity: There is an exhibition of rare and unobtainable items in the Isle of Merriment which can only be accessed by players during test servers. Since the first public version of this item exhibition (WU19), we can see a Throwing Star of Sula on its southwest corner.
A Throwing Star of Sula can be seen in the Isle of Merriment
(screenshot taken on the TS for the SU21)