Zaros brought many secrets with him to Gielinor. Some were from Infernus, some from other worlds he had visited, and some - I think - were of his own devising. He understood the physical needs of the humans and shared the knowledge of how to provide for them with the church and ultimately the whole empire. Though a Pontifex, I was responsible for civil engineering in the whole western half of the empire, and had precious little time for preaching.
Food and water were the most important concerns. The humans of the dark age lived in rural communities of no more than a few hundred, and were able to survive through hunting and subsistence agriculture. The great cities of the empire could not create enough food this was because the population density was so high. Zaros taught the humans how to farm more intensively, and how to replenish the salts of life in the soil through crop rotation. Late he devised a method of deriving salts artificially through the use of furnaces powered by demons. Water was as crucial, and as the cities grew they needed more than the nearby rivers and springs could provide. Great artificial rivers called aqueducts were erected to carry water from sources far outside the cities, both for human consumption and for irrigation, allowing the farmlands to expand even further. As the water became even more plentiful there was excess sufficient for luxurious baths, at least for the wealthy and worthy.
The disposal of waste became the secondary priority as the provision of plentiful food and water allowed the population to reach never before seen heights. Though the humans - like animals - seemed content to simply let their waste pile up around their dwellings, Zaros determined that this was a source of sickness and devised a system of cloacas - channels similar to aqueducts - to carry the water safely out of the cities. In later times an industry grew up around the processing of this waste into fertilisers, tannins, explosives and other useful compounds.
Another of Zaros's gifts was a shared language - that of the ancient Infernals - that was mandated for all humans and quickly supplanted the many and varied tribal dialects. This, together with a shared currency, allowed for communication and trade between the smaller communities, allowing for prosperous multiplication of wealth. The temple schools that taught the language also expanded to the teaching of the sciences. Though Zaros instructed the church to teach the whole population to read and write, it was economically impractical to do so for humans working in agricultural or industrial labour. Nevertheless the empire had a level of literacy that eclipsed that of any other god's people, with as many as one in five able to read and one in ten able to write.