Many people in the galaxy fear droids, what with the memories of the Clone Wars still fresh in their minds. Far more terrifying than battle droids were assassin droids, independently programmed mechanical killers that had no masters. IG-88 is a battered chrome war droid who has become a bounty hunter, and answered Darth Vader's call to capture the Millennium Falcon during the events surrounding the Battle of Hoth.
Eager to find Luke Skywalker after the Battle of Hoth, Darth Vader assembled an motley assortment of bounty hunters to track down Skywalker's allies. Among the hired guns gathered aboard Vader's Star Destroyer was IG-88.
Darth Vader offered a substantial reward to the bounty hunter who tracked down the whereabouts of the freighter Millennium Falcon and its crew. Of the hunters gathered, though, it was not IG-88 who successfully tracked the quarry.