The Sleep Spell was the curse Magus cast on the surviving members of the Wyvern Clan following the events of the Wyvern Massacre in A.D. 994.
The Magus used first this spell from the Grimorum Arcanorum on the gargoyles later known as Hudson, Brooklyn, Lexington, Broadway, and Bronx after believing that Hakon had succeeded in slaying Princess Katharine.
The Magus was stunned when Katharine returned alive and that Goliath had actually saved her. Immediately regretting his decision, he was unable to reverse the spell, as earlier that night Hakon had burnt the page with the counter-spell upon it. He did, however, at Goliath's request, place the same spell upon the gargoyle leader, who had at that point become too grief stricken to live (or even consider having a Wind Ceremony for the massacred gargoyles), though before asking the Magus to cast his spell one more time, he did manage to ask him and the Princess to care for the gargoyle eggs in the rookery. ("Awakening: Part Two") 
- Dormiatis dum castellum super nubes ascendat. 
- May you sleep until the castle rises above the clouds.
Behind the Scenes
As this was the first example of mortal magic used in the series, Greg Weisman wanted the rules of mortal magic to be well established for the course of the series. Basically every mortal spell needs a time limit attached to it, and for the more powerful spells (such as this or the Stone By Night Spell) this condition can be satisfied with an event so improbable that it can effectively equate to 'until Judgment Day.' According to Greg, from the Grimorum's point of view, until the castle rises above the clouds "meant 'until hell freezes over' or some equivalent."