You would think it's the conjunction of "there" and "will", but it's actually "there" and "shall".
It was written for the era before we started using "will", so technically it's extinct in modern english, except in legal documents, along with "shall". That's why a modern spell checker won't recognize it and a good one will tell you that it's grammatically incorrect.
The funny thing is, nothing is differentiated between them when using the word in speech so the word has been passed down verbally through generations from the use of "shall" to the use of "will".
Today, people say it conveniently without recognizing the specific application of "will" or "shall" before a sentence and is not only the reason why "shall" has become used less and less, but also the reason why we still say the word "there'll".