Closed gene pool tribe after 800 years?

Question

I'm leading my D&D party through an undiscovered land, where, as far as they know, no one comes back from. I want them to encounter an isolated tribe of as few people as possible -around 30 maximum. I figured they'd be what's left of much larger group of people after around 800 years. The lore is that a magical cover had been set around the terrain to hide something inside. Whoever crosses it, soon dies. The area covered is vast (~1000km radius) and so a few cities and villages have been under the spell as well.

I'd want them to be the citizens of one of them after 800 years of being cut off. Is this viable?

Their bodies would, I assume show some deformations , they would be weak and prone to illnesses, right?

They'd be illiterate because of lack of 'the elderly' in recent generations (they'd die pretty young due to the illnesses and genetic illnesses). Also I see them as childish and utterly immature for the same reason: their parents would die too young to show them how an adult behaves and well, not knowing it either since they (the parents) would die before 25 when the brain matures.

Since I added some assumptions I repeat the question: is it viable? Where am I mistaken?

Answer 1

I don't get why you would worry about the plausibility of an isolated tribe in a world where you have undead, mages, dragons and associated.

However, what you describe is more or less the situation of the tribe on North Sentinel island

North Sentinel Island is inhabited by the Sentinelese. Their population was estimated to be between 50 and 400 individuals in a 2012 report.[2] India's 2011 census indicates 15 residents[39] in 10 households, but that too was merely an estimate, described as a "wild guess" by the Times of India

They live in isolation from the outside world for a few centuries, and are all but childish and immature, they are capable of defending their territory from intruders and to scavenge shipwreck for harnessing metal. They are genetically frail due to the lack of diversity, but only if exposed to outside threats or sicknesses.

Being illiterate has nothing to do with the size of the population: large populations have thrived without having a written language.

By the way, considering that humans can reproduce already at 10-12 years of age, dying at 25 is well past that age. Some grandparents could still be around.