I am trying to look for cited source who is said to have been written by Tulle. The text that Tulle is cited is about the death of Romulus when he disappeared and made to a god. Any hints?
"Tulle" is Cicero. His full name was Marcus Tullius Cicero. Now we're more used to calling him by his cognomen, but formerly he was referred to by his nomen, Tullius, or Tully or Tullie in English. In fact we still do this for Virgil (Publius Virgilius Maro) and Horace (Quintius Horatius Flaccus), but it used to be the opposite way around. English-speakers called Virgil and Horace by their cognomina, "Maro" (or Anglicized to "Maron") and "Flaccus".
Presumably this was influenced by/related to the French tradition where he was also called Tulle or Tulles. I know of at least one medieval French text that calls him "Tulle" (John of Ibelin, Livre des Assises, ed. Peter Edbury, Brill, 2003, pg. 717).
I'm not sure why or when we switched, but I do know there's a book about how people read Cicero in early modern English, which may be helpful - Master Tully: Cicero in Tudor England by Howard Jones (Nieuwkoop, 1998).
Cicero talks about the disappearance of Romulus in De Re Publica, so that's probably what you're looking for.