The city which the Sigilyph guard is no more, for although these pokemon were never defeated by invasion, they were unable to prevent an ecological catastrophe. Environmentally destructive farming practices turned the land which these pokemon guarded from a great metropolis to a shrinking one, and finally, when the last people left, into a ruin absorbed by the desert. But the watchful eye of the Sigilyph continued to protect this ghost town from thieves, and today it is one of the best preserved archaeological sites in Unova.
It is fortunate for the cause of human knowledge that the Sigilyph are psychic pokemon, for this meant they were able to read the hearts of archaeologists and determine that most of them lacked any ill intent. A few individuals – those motivated by fame or those wishing to steal the Sigilyph city's treasures for the prestige or payments of museums in distant towns – did meet the fate of many aspiring robbers, driven away by fierce winds and invisible walls. But those willing to dig with respect for these pokemon and the civilization which they have so long protected and wishing to advance the cause of knowledge saw the Sigilyph use their powers to aid their efforts, clearing their eyes from sandstorms and using telekinesis and ancient knowledge to lead them to parts of their city which would otherwise be left forever undiscovered.
When the city became a protected archaeological site, many of the Sigilyph flew away to explore, battle, and live lives that had been on hold for ages, for they considered their mission complete. Others remain, for they do not completely trust human institutions to keep their wondrous ruin safe from harm, or simply because, although it was abandoned by humans centuries ago, the Sigilyph still consider it home.