Seventh Ark

The Seventh Ark is a dummied out location in Final Fantasy XIII. It was meant to be accessed via a teleport gate in the Orphan's Cradle after completing the dungeon.

The dialogue before the area's boss would be dependant on the current party leader.

Datalog
A Datalog entry exists across all versions of the game, although only the Spanish version is known to have a complete entry for it as the English one says: "PLACEHOLDER — TO BE TRANSLATED LAST AS THIS IS GOING TO BE DELETED ANYWAY".

The Spanish entry for the Seventh Ark translates to:

Locales — The Seventh Ark= ''One of the military installations of which the legends of Gran Pulse speak. The Fifth Ark is clearly an arsenal ready to combat invaders; the Seventh, instead, has as its principal function to awaken the hidden power of the l'Cie.''

The entry for Nemesis, the fal'Cie which oversees the Seventh Ark, also exists but has the same Datalog issue like the entry for the location.

The Spanish entry for Nemesis translated to:

The Fal'Cie — Nemesis= ''Fal'Cie lord and master of the Seventh Ark. Its work consists of harassing the l'Cie in the middle of cruel tests and obligating them to surpass their own limits.''

Apparently it trains them to fight against someone, but it is not known who.

Nemesis and its Ark already existed in the times before the construction of Cocoon.