Tupile

Tupile was a secretly held region within the known universe, peripherally regarded as a single world and called the sanctuary planet, though it was in truth a group of several worlds. Its exact location was known only to the Spacing Guild and was never disclosed to other groups or organizations, and that secrecy was the whole basis of its function. Under the Faufreluches of House Corrino, Tupile served as the place of refuge for defeated and renegade Great Houses sent into exile. (Dune; Dune (appendix))

A hidden region#

Tupile was a secretly held region within the known universe. Although it was peripherally considered a single world, the sanctuary planet, it was actually a group of several worlds. The exact location of Tupile was known only to the Spacing Guild, and the Guild disclosed it to no other group or organization. That absolute secrecy was what made the region useful as a place of hidden refuge. (Dune (appendix))

Sanctuary under the Faufreluches#

During the time of the Faufreluches under House Corrino, Tupile was used chiefly as a place of sanctuary for defeated or renegade Great Houses of the Imperium when they were exiled. The glossary to Dune describes it as the sanctuary for defeated factions of the Imperium, kept inviolate under the Guild Peace, and in the novel Duke Leto knows that a House which loses everything can go renegade and flee there. The hidden region thus stood as the last refuge for the losers of the Imperium's feuds. (Dune; Dune (appendix))

After the Atreides#

When Paul Atreides took the throne, the defeated House Corrino went into exile on Salusa Secundus rather than to Tupile. Frank Herbert's later novels say little more about the sanctuary worlds. (Dune; Children of Dune)