The Deluxe version of the Great Darkness Saga reprints the earlier stories from when Paul Levitz returned to the Legion as writer in Legion 284. The Saga doesn't begin until 290 (although there was a backup prelude story a few issues earlier), so you have 6 issues of build up to understand what happened before with Legion 284-289.
The thing is, I'm not sure if the Great Darkness Saga trade is still in print. I see it at Amazon at a higher price than normal. If you can find it, I'd grab it now because who knows when DC will reprint it.
https://www.amazon.com/Legion-Super-...2004399&sr=1-1