Talisman: Digital Edition

The description of the achievement could be a bit clearer, but the Pied Piper reward can be won by having the most followers during a game.

This is highly dependent on the adventure cards you draw obviously. But playing as the Vampiress from the Highlands expansion (unless I'm mistaken) will make that task easier, since you may steal followers from other characters by simply landing on their space (no combat is necessary).

You need to actually finish a game, win or lose, to get any kind of reward.