The Auld Lang Syne Spell was a spell cast by Chlorhydris upon the Smurfs during the events of "Days Of Auld Lang Smurf". The spell, which can only be cast around the time of New Year's Eve, causes those on whom the spell is cast upon to see and interact with physical hallucinations of people (usually loved ones) who have passed away, making the victims believe that they have been brought back to life somehow. However, the spell can also kill those who experience the hallucinations when they vanish at midnight on December 31, which makes the victims go into a painful state of shock.
In "Days Of Auld Lang Smurf", most of the Smurfs in the village were affected by the Auld Lang Syne Spell when they saw their parent Smurfs mysteriously brought back to life. However, Smurfette was not affected by the spell since she had no parents to begin with, and neither were Polaris Psyche, Baby Smurf, or the Smurflings, who still had existing parents. Because of this, they were able to see that this "resurrection" was nothing more than an illusion since they were not able to see, hear, or even touch the hallucinations. Grouchy, who originally became a victim of the Auld Lang Syne Spell, was able to break free of its influence when he realized that the "resurrected" Muscles Smurf, his father, still loved the other siblings of his family more than him, and thus hated him.
After the unaffected Smurfs found out about the spell and its intended effect and outcome, Baby Smurf used a counter-spell on the affected Smurfs that altered the appearance of the hallucinations so that the affected Smurfs would see them as hallucinations, which would prevent them from being shocked to death around midnight on December 31. Despite Chlorhydris' intentions with the spell, the Smurfs that were affected by it would be moved to reforge their relationship with Papa Smurf as being their one and only Papa Smurf.