All we know is that it's a terrifying being trying to alter our world irrevocably.
The Eroder is the boss of the Close Encounter of the Terra Kind collaboration quest in Final Fantasy XV. Noctis and Sarah face it at the end of their quest. The Eroder is a mystical being that absorbs data from the environment, copying it and growing stronger. It wants to remake the world, and thus the Great Tree summoned Noctis and Sarah to slay it.
The Eroder initially poses as a Hiso Alien, but Sarah senses something amiss and shoves Noctis aside, being knocked down by it in the process. The Eroder copies Sarah's appearance and powers and now appears similar to her with an inverted color scheme. The "whites" of its eyes are black and it has red scarring around its right eye. Occasional red data flickers across her body. The Eroder considers itself to be a god destined to rule over the Great Tree's realm. It is power-hungry, reveling in the power it copied from Sarah.
The player can view the boss's stats by scanning it in Wait Mode. Magic, elemental damage, status effect moves, point-warping, Armiger and summons are not available. The Eroder does not have a Bestiary entry.
This power, this wonderful power... It's beautiful!
The boss battle is fought in two phases. After the Eroder loses half its health, the player gets a cut scene and the battlefield turns red.
The player can use daggers, a shortsword, a polearm or a royal arm (if they did the item delivery quest back in the Hiso Alien village). Attacking the Eroder's behind triggers blindside attacks and link-strikes with Sarah: there are two link-strike animations and they can trigger with any weapon, unlike in the main game where link-strikes are triggered by specific weapon types. Sarah can't go into Danger state, but Noctis can; if that happens, Sarah can rescue him.
The Eroder has copied Sarah's abilities, and thus uses the same battle animations. It attacks with its sword when near to the party. Shades of the Eroder, called ???, manifest to attack the party. The ??? enemies sometimes resist some/all weapons. They can disappear and reappear, and can be defeated to get rid of them, although more will keep coming. The ??? send gravity balls that float about the battle arena and damage those who touch them and wield a glowing red energy sword, which they slam to the ground in front of them.
After the Eroder loses half its health, the battle enters the second phase. The ??? no longer appear, but the Eroder itself now uses their gravity sphere and energy sword attacks and warps around the battlefield (like Noctis does on the field; the Eroder doesn't warp-strike), making the player chase after it.
The battle arena has no point-warp spots.
I shall delete you.
The battle is not hard, considering the player can't save during the quest, but one can stock up on items in the Hiso Alien village prior to the boss battle by repeating the item delivery quest.
The player should aim for blindside attacks to trigger links with Sarah. When the Eroder attacks with its sword, holding the defense button evades the attacks. Sarah's Salvation is good and makes the player invincible for the animation duration. It is also cheap to use, costing only one tech bar. Salvation can one-shot the ???, but as soon as the player kills one another will appear.
In the second battle phase, the Eroder starts warping away from the party. The player can warp-strike it to close the distance and to deal damage. The Eroder can now avoid Salvation by warping away from where Sarah deals the attack, making it less reliable. The player should still go for the blindsides even if it is harder now that the Eroder can warp.
Owing to it copying Sarah's form and abilities, the Eroder shares the same voice actors as Sarah in all languages, although with a slight echo and mechanical reverb to her voice.
- The Eroder's attack animations are recycled assets from the main game. The gravity sphere attack is from Prompto's Technique and the energy-sword slashes are from Ravus, though red rather than Ravus's purple. The warp animation is the same as Noctis's warp on the field.
- The red version of the battlefield somewhat resembles the light in the "red world" seen in pre-release material, which appears to depict the long night.
- When confronting the Eroder's second form, Sarah asks if it is a ghost, and Noctis replies that it's "not the friendly kind", alluding to Casper the Friendly Ghost.