The mechanized civilization of Solheim is presumed to have originated in the Duscae and Cleigne regions. Exactly when the civilization rose and fell, however, remains a subject of much investigation.The Rise of Solheim lore exposition in Final Fantasy XV: Episode Ardyn
In Cape Caem's secret harbor, next to the photograph of Regis Lucis Caelum and his retinue, is a newspaper with a headline: "Amazing 1st Civilization Discovery." This likely means Solheim. During the Adventurer from Another World quest, Ignis Scientia notes that the teleporter from Eorzea looks strikingly similar to Solheim's magiteknology.
According to the ancient chronicles, Solheim ruled the lands of Eos during an era of peace and prosperity when all six Astral gods looked over Eos together. It excelled in machine technology, such as like airships and magitek armor. The center of their empire was in the region between modern day Duscae and Cleigne.
Solheim worshiped the God of Fire, Ifrit, who had granted them the knowledge of fire and thus aided in their technological advancement. According to legend, Ifrit first bestowed his "burning wisdom" upon a man who later sat the throne of Solheim. The people's worship of Ifrit tied into the civilization's views of life and death; fire represented life and was a sacred symbol that had temples built in its honor. Water represented death and Solheim paid tribute to the dead at the Vesperpool through the Steyliff Grove. Solheim created unparalleled architectural feats, including the still-standing Costlemark Tower and multiple subterranean ruins; one of the most famous is the Pitioss Ruins, a dungeon where the laws of physics were distorted into a training ground for an unknown purpose.
At its peak, the hubris of Solheim fueled its citizens' scorn of the gods and turned them against their benefactor, Ifrit, and the rest of the Astrals. Gentiana tells Noctis and his retinue how Omega is a weapon built by man to fell the gods. Omega appears from a portal that depicts a symbol associated with Solheim, suggesting it was built by Solheim to kill Astrals.
The lore in Ifrit's bestiary entry is slightly different between different translations. The Japanese version says Ifrit was the King of Solheim. The English description says Ifrit's "once-loyal subjects turned against him", but doesn't mention the corruption of his body and mind. The German version says Ifrit was once "a lover of mankind" whose gift of knowledge led to the success of the Solheim civilization. The lore exposition in Final Fantasy XV: Episode Ardyn posits that the man whom Ifrit had granted knowledge of fire became the first king of Solheim.  The German version of the Chapter 12 scene regarding the Astral War posits that Solheim tried to destroy the gods, and that Ifrit in turn sought to destroy all of mankind and the planet of Eos itself.
Ifrit turning on Solheim led to the other Astrals joining the war to quell Ifrit's wrath and save what remains of mankind from being consumed by fire. Ifrit was killed and Solheim's civilization collapsed, leaving the planet devastated. According to legend, Ifrit's corpse was laid to rest within the depths of the Rock of Ravatogh, the enormous volcano in what modern times is the southwest of Lucis.
The appearance and spreading of a mysterious parasite concurrently fueled the war, and killed millions. Labeled the Starscourge, it created daemons. This eventually led to two mortal bloodlines being granted special powers by the gods: a king granted power to fight the daemons and the gift of the Crystal to protect, and an Oracle granted power to heal and to commune with the gods, serving as liaison between the Astrals and mankind.
The Niflheim Empire's Aldercapt dynasty, helmed by Iedolas, intends to recreate the Solheim Empire under their control. Many ruins of Solheim origin remain on Lucis, including Steyliff Grove, Costlemark Tower and Pitioss Ruins. Niflheim investigates them, and according to Aranea Highwind, captures daemons from them. It is ambiguous whether Solheim used daemons to power their magitek the same way Niflheim does during modern times, but the Solheim ruins are only open at night. Niflheim has begun to use magitek and seeks to destroy the Astrals, seemingly following in Solheim's footsteps.
Behind the scenes
The Final Fantasy XV E3 2013 trailer says that "Niflheim has already waged war for crystals in Solheim, Tenebrae and Accordo", suggesting Solheim was to be contemporaneous to the current Final Fantasy XV timeline. In the final game Solheim is an ancient civilization and never had a Crystal.
The Complete Official Guide posits that the Astral War was fueled by the spread of Starscourge that killed millions and led to the fall of Solheim:
Ancient history is marked by the prosperity of the civilization of Solheim. When this civilization was at its peak, the betrayal of one of the six Astral gods, Ifrit, led to a conflict known as the Great War of Old. This was fueled by the spreading of a mysterious parasite, causing the death of millions, and ultimately the fall of Solheim.The Complete Official Guide pg. 318
However, the game itself gives no timeline on when the plague appeared, although the German version posits it originates from the Meteor of the Six. Shiva never mentions the plague in her telling of the war in Chapter 12, and the Final Fantasy XV Scenario Ultimania first mentions the plague "spread due to a parasite" around 2,000 years ago. The Astral War and the prosperity of Solheim seem to be longer than 2,000 years ago, as they don't have a presence in Final Fantasy XV: Episode Ardyn – Prologue where Ardyn and Aera also explore old ruins, implying a fallen civilization.
Although the Japanese version of Ifrit's bestiary entry in Final Fantasy XV says Ifrit was the king of Solheim, Episode Ardyn Prologue posits Lucis was to be the first kingdom of Eos, its sovereign to be chosen by the gods.
Markings associated with Solheim
Solheimian technology is associated with a circular marking that glows red, as seen with the warp pads in Costlemark and Steyliff, the action pads in Pitioss Ruins, and a portal from which Omega emerges into Insomnia. In the final maze portion of Costlemark, the player can find the correct path by looking for a circular mark on the floor at the final junction.
The symbols on the sealed doors, as well as the shapes of the seals themselves, appear similar and also glow red. However, the seals were said to have been erected by a king and an Oracle, and also resemble the markings present on havens, places blessed by the Oracle to ward off daemons.
A symbol very similar to the Solheimian "action pad" also appears in concept art for a final and unused Rursan Reaver enemy in Final Fantasy Type-0, where the circle represents the "eye" of Etro's gate, or the portal to the afterlife.
Etymology and symbolism
Heim is German for "home".
The theme of light and darkness is prominent in Final Fantasy XV, with Bahamut, the Crystal and the souls of mankind—and thus the kings of Lucis and the Oracles—representing the powers of light, and Ifrit, Starscourge, daemons and thus Ardyn Izunia representing darkness. The world itself is called Eos. In Greek mythology, Eos is a Titaness and the goddess of the dawn.
Solheim, the "home of the sun", was the civilization Eos had before the darkness was brought forth. Though it collapsed by the powers of the dark, Bahamut gave mankind the means to return light to the world, which became known as the prophecy of the King of Light.
- Final Fantasy XV Scenario Ultimania Translation Project — History of Eos (Accessed: May 02, 2018) at Medium
- Bestiary: 古代文明ソルハイムに君臨した王でもある
- Some claim that humans discovered fire in the Succarpe region even before the dawn of the Solheim civilization. According to legend, the fire god Ifrit first bestowed his "burning wisdom" upon a man who later sat the throne of Solheim.
- Shiva: Doch die Harmonie sollte nicht ewig halten. Die Zivilisation Solheims erreichte ihren Höhepunkt - und trunken von Hochmut versuchten Menschen, die Götter zu vernichten.
- Shiva: Dieser Verrat brach Ifrit das Herz und entfachte seinen Zorn. Blind vor Wut wollte er diese Welt mitsamt der Menschheit einäschern. Und so… blieb dieser Göttin keine Wahl. Sie musste gegen Ifrit Kämpfen, um Eos zu schützen.
- "Ancient history is marked by the prosperity of the civilization of Solheim. When this civilization was at its peak, the betrayal of one of the six Astral gods, Ifrit, led to a conflict known as the Great War of Old. This was fueled by the spreading of a mysterious parasite, causing the death of millions, and ultimately the fall of Solheim." The Complete Official Guide pg. 318
- Final Fantasy XV Ultimania Scenario Side translations (Accessed: February 19, 2019) at The Lifestream