The Seitengrat is an invisible bow in the Zodiac versions of Final Fantasy XII. It is one of four secret equipment pieces added in these versions, including the ultimate one-handed melee weapon Great Trango, the lethal status combination inflicting weapon Bonebreaker, and the high evasion all element-absorbing shield Gendarme.
The Seitengrat is by far the most powerful weapon in the game, and has double the range of all other long-range weapons, which makes it an easy choice for clearing out regular mobs. Its drawback is that it is a two-handed weapon, and while it also has the highest evasion of all weapons, the Great Trango (which has 0 evasion) is one-handed and so can be equipped alongside the Gendarme shield which compensates and then some.
The Seitengrat can be obtained when the player can board the Skyferry. Entering the deck of the Skyferry offers a 1% chance for an invisible chest to spawn, that has a 20% chance of containing an item, and if the player has the Diamond Armlet equipped, the chest has a 5% chance of containing the Seitengrat. With this 0.01% chance every time the deck is entered (assuming the player knows of the hidden chest), the Seitengrat was intended to only be gotten by fluke or serious persistence, however, with RNG manipulation one can guarantee the chest spawns and relinquishes a Seitengrat.
If the player acquires the Seitengrat, an arrow will appear in the sky in the Cerobi Steppe suspended a few feet in the air.
It is possible to guarantee that the Seitengrat will spawn on the Skyferry by exploiting the game's random number generator (RNG) in The Zodiac Age version, with various methods such as this in this YouTube guide. The player should create a new save file in Nalbina Fortress at the beginning of the game as Reks, and then load their main save file. In their main save file, the player should set Vaan as the main party member and equip him with a Diamond Armlet, and then visit Rabanastre and take the Skyferry to Nalbina. From there, the player should enter the deck, and then close their application.
The player should load up their newly created save file at the beginning of the game in Nalbina Fortress as Reks, and cast Cure on themselves, until the amount cured is three specific numbers in the exact order: 88, 97, then 90 (Depending on the player's PlayStation 4 model, they may need to instead reach a different set of numbers: 97, 94 and 95; or 88, 94, 92, 92, and 96, providing the other methods do not work). The player should quit to title screen, load the auto-save made in the Skyferry, and the invisible Seitengrat chest will have spawned. The weapon can be obtained repeatedly with this method up to 99 Seitengrats.
For the Steam version, the player may opt to use the third party tool Cheat Engine to force the chest to spawn with the desired item inside.