riddle - I am the fifth of my kind

Sit down, gather round, listen to my tale

Of a time when humanity would finally fail

To conquer death like it always had,

we've died before, but this time was bad.

In threes they came, spreading like plague

We know many died, but the numbers are vague.

After many a year, the sick called "so long!",

Leaving only the lucky who managed to stay strong

Then I came I came along, and spirits were raised.

A new era was born, you'd be amazed

To live in my kingdom, when life was the best,

When the whole human race put their limits to the test.

Should be pretty easy... So what am I?

Answer

What is...

the renaissance?

Sit down, gather round, listen to my tale

Of a time when humanity would finally fail
To conquer death like it always had,
we've died before, but this time was bad.

The black death.

In threes they came, spreading like plague

The black death was a combination of three plagues.

We know many died, but the numbers are vague.

The amount of people who died are counted in millions, not very precise.

After many a year, the sick called "so long!",
Leaving only the lucky who managed to stay strong

The survivors. The people afterwards are believed to be better.

Then I came I came along, and spirits were raised.

A new era was born, you'd be amazed
To live in my kingdom, when life was the best,
When the whole human race put their limits to the test.

The Renaissance. The black death was in the late Medieval era.

Hint:

Post hoc, ergo propter hoc could be referencing the fact that people attribute the fact that the humans after the black death were better(X) to the black death(Y).