Few would disagree that Nex was, if not is, Golarion's greatest wizard. His arcane influence on the world- and in some ways, beyond- is contested by Old-Mage Jatembe alone, though the contributions Nex made to the world were often as happenstance as they were calculated. The archmage wizard-king is recognized as the tremendous force responsible for a nation of great impact. The extent of that greatness only comes into full relief four millennia after his disappearance from the Material Plane, and that greatness has limits.
The sprawl of Nex's consequence is a different story. With each year that passes, the wrinkles of his ambition and ego mark Golarion. Ironically, the country of his namesake looks the most weathered through time by his actions. Nex, the country, is wound of ancient wonder and woe alike.
Nex presents a surreal romance to newcomers. Quantium, a capital whose handsome visage flows from subterranean depths to skies of unusual color, brandishes a dizzying array of wonders conjured from hither and thither. Unknown is less showy, but an intrepid traveler can find a world within themselves as complex as the enchanted warrens of the Unknown palace in the capital; all it takes, in the alchemical city, is cleaning a plate of Ghoran cuisine and reaching the bottom of an exceptionally potent glass. Few who live in the military hold of Unknown care if it isn't the nation's most charismatic city. It's formidable, and because it's formidable, so is Nex.
Staying in the nation exposes its preexisting cracks. Everyone knows not to traverse Nex's countryside, a scourged area nearly as dangerous to traverse as the Unknown, blasted from the archmage's war with Geb 4,000 years ago. Dangerous miscreants, harsh wild things, cold desert-like nights, and waywardly inconsolable fleshforged titans menace the countryside. What few will tell you in the countryside is what nobody will tell you in Oenopion: if you're new to the city, there's a better chance you'll vanish there than in the wastelands.
The capital is built of secrets reaching a bureaucratic tipping point. The people of Quantium fend for themselves for the sake of provoking- or preventing, depending on whom you ask- the reignition of war with Geb. Each secret revealed, ancient or new, weighs the scales more heavily to the former. Ecanus is supposed to defend its two fellow cities from threat, but it's a literal festering wound of a city, which only causes the wider nation to bleed internally. What dealt the blow that led to the Evisceration of Ecanus is unclear, but if it wasn't Geb's doing, then Archmage Nex's dream is more self-sabotaging than his rival necromancer could ever have wished.
Oenopion poisons not only itself with the great, sentient mass of oozes known as Unknown beneath the city, but also assists in a wider poisoning of the region's waters with irrigation that leaches water from the rivers to its south and north. This is nothing compared to the Miasmere, a horrendously polluted bay made so by years of willful neglect of Quantium's arcane activity, which contaminates an water entering the nation from the Obari ocean. The Miasmere runs westward down the Elemion and Ustradi rivers, poisoning them both in the process. Unknown is a dumping ground for prisoners and enemies of the nation, but more than a few officials bringing them there end up trapped on the island with Nex knows what else.
In the Mana Wastes, the last gasps of Nex's continued expansion (which started the war with Geb over and age ago) are fading as the technocratic Grand Duchy of Alkenstar forms into a territory of its own and crawls out of Nex's grasp. In the capital, the last person to know the archwizard- and the oldest member of the ruling Council of Three and Nine- has gone missing. The nation's leadership fights a newly cold civil war as it prepares for the resurgence of a literal, deeply scarring ancient one.
People come to Nex because it's amazing. Such a place reforms those who visit. How could it not? The process of creating his dream to the fullest of his wishes didn't change Nex himself; it exposed him. The nation is a fertile ground for everyone who follows him, to discover, and, even in unexpected ways, to grow. Nex might have created much in the world, but the people who traverse his domain always find themselves asking: what good did he actually accomplish? Then they do the magical thing that Nex was meant to do, and they accomplish something new.