Mirlantha is a low-magic fantasy setting, dominated by humans, with a technology level equivalent to medieval Europe,
Key assumptions of the setting include:
Human-dominated - It is assumed that PCs will be human. Elves and lizardmen are known to exist (although in a different form to the standard D&D elves) but they are rarely seen by most humans. Other intelligent races such as dwarfs, gnomes and fairies are only known from myths and legends and may never have existed.
Restricted magic - The general population knows that elves are capable of using magic and that they produce some magical items for use by the wider population. Humans have some capacity for learning magic, but it is a long and difficult path. The level for arcane spells is increased by 1 i.e. cantrips use level 1 slots (and so are limited uses) and PCs will not be able to cast level 9 spells under normal circumstances
Points of Light setting - Although Mirlantha has been settled by people, much of the land is still wild, untamed and full of monsters. Travel can be slow and dangerous, even in relatively civilised areas.
New settlers in an ancient land - Although modern humans have only been in Mirlantha for a few generations, there is evidence of ancient civilisations scattered around the land, in the form of ancient monuments, half-buried ruins and mysterious tombs.
Pre-industrial technology - think hand tools, horse-drawn carriages, windmills and waterwheels, with the rare addition of magical effects. Gunpowder, combustion engines etc haven't been invented yet.