Not as in Darth Vader, Dark Lord of the. Hm. If Darth Vader were a Shakespearean character, who would he be? I’ll get back to you on that one. Do you have any ideas? But I digress… on to the word!
sith (conj.) IPA Pronunciation: 
since
HAMLET
…I do not know
Why yet I live to say ‘This thing’s to do;’
Sith I have cause and will and strength and means
To do’t.
– Hamlet (IV.iv)
Our modern word “since” is used a conjunction as well as an adverb and a preposition, but all the uses I found of “sith” are a conjunction, but there may be exceptions. But why bother? Students these days don’t learn these parts of speech in school. They are taught, though soon forgotten.