You brought the scumbag to justice. Magpie cheated and deceived many a stalker, and now his time has come.
This achievement is earned for completing the side quest In Search of Magpie regardless of the outcome. The achievement is earned once Flint's true identity has been revealed to either Gonta, Loki or Shulga.
There is no reward for obtaining this achievement. In-game text suggests that Magpie's stolen loot would have been deposited into the player's locker in Yanov, but in-game the achievement isn't associated to any reward.