The Lost Crew

The lost crew is a tall tale since the dawn of space travel. It's space crews that got lost adrift in the space, but didn't die. Maybe they fell through a rift in space time. Only knows that they appear without warning or cause to bring other ships into the Void with them to add to their fleet.
Strangely enough this tale echos through all races that have been into space. Some say they saw these lost ships floating between realms.
Though what's worst they say they see different races space crafts floating with them....and they didn't resemble any know races ships at all.

Summary

It's unknown how or what triggers the signal that spawns a lost ship, but when it does all hands better be prepared to get away, as no space is safe they freely travel between dimensions through only stay for a limited time before slipping back into their subspace.
Humanity has lost several ships to these creatures and what was more frighting is that the ships lost were now being seen chasing our force.