When a concert (or other event) is a "sell out", it means that all the tickets have been sold. In this instance, it was a "huge sell out", which probably means that there was a very great demand for the tickets (perhaps they were all sold within the first few hours).
"Inevitable" here is obscure. It may mean "exactly as she ought to be".
An "illusion beyond imagination" is "something that nobody could have imagined". In other words, it was "beyond the speaker's wildest dreams.
All the best,
- For Teachers