Midlight's Deep

The Midlight's Deep, also known as Deep Dungeon, is a location in the world of Ivalice. A dark dungeon, it contains many strong monsters.

Final Fantasy Tactics
In Final Fantasy Tactics, Midlight's Deep serves as a sidequest. Located east of the Port City of Warjilis, it consists of ten levels of caves, each of which is pitch black until lit by the crystals left behind by enemies when they are killed. To progress to the next level down, Ramza Beoulve must have an ally with Move-Find-Item search for a passageway which leads to the next level. This passage is always hidden in one of five different positions on each floor. After finding the next passage, the heroes must then defeat all enemies on the current level to complete the current battle and unlock the next one.

The tenth level down contains the battle with the Lucavi demon Elidibus, during which Ramza is aided by Byblos. If Byblos is not killed during the battle, he will join Ramza after the fight is over.

The Levels
The names in the PlayStation One and the PlayStation Portable versions.
 * NOGIAS - The Crevasse
 * TERMINATE - The Stair
 * DELTA - The Hollow
 * VALKYRIES - The Catacombs
 * MLAPAN - The Oubliette
 * TIGER - The Palings
 * BRIDGE - The Switchback
 * HORROR - The Interstice
 * END - Terminus

Trivia
The name from the PSOne version pertain to the movie Apocalypse Now. MLAPAN and NOGIAS are Napalm and Saigon spelled backwards

Final Fantasy XII: Revenant Wings
Midlight's Deep is a secret dungeon only available in the US version. The final boss that awaits the party within the dungeon is Yiazmat.