Here is the synopsis for it.
SUPERGIRL ANNUAL #2 Written by STERLING GATES Art by MATT CAMP Cover by AMY REEDER & RICHARD FRIEND In a special story that has direct ties to the regular monthly SUPERGIRL series, the Girl of Steel visits The Legion of Super-Heroes on Halloween in the 30th century! But Halloween proves to be just as spooky a holiday a thousand years from now as it is today, because a horrible new threat has awoken thanks to Supergirl's arrival. And that horrible new threat's name is - Satan Girl! Brainiac 5 has a plan, but it may come at the cost of shattering his potential romance with Supergirl!
It comes out on October 27!
This is the first time Brainy meets Supergirl, although not the first time Supergirl has met him. Don't you love time travel paradoxes.