What are the "lovely tales" in Keats' "Endymion"?

Question

What is Keats saying in the last three lines here?

And such too is the grandeur of the dooms
We have imagined for the mighty dead;
All lovely tales that we have heard or read:
An endless fountain of immortal drink,
Pouring unto us from the heaven's brink.

_{John Keats (1818). Endymion, Book I, lines 20-24.}

Is Keats referring to "tales" of an afterlife / immortality in heaven? If so, why are those "tales" an "endless fountain"? Because there are so many different views on life after death?

Or could he perhaps be saying that "tales that we have read" grant their authors immortality?

And what might be the significance of the aquatic imagery ("fountain", "drink")?

Answer 1

Lines 1–33 of ‘Endymion’ form an introduction to the story, which starts at line 34:

Therefore, ’tis with full happiness that I
Will trace the story of Endymion.

The only way to understand the word ‘Therefore’ in line 34 is that lines 1–33 consist of an argument as to why Keats should be happy to tell the story of Endymion. What is this argument? Well, first, ‘a thing of beauty is a joy for ever’, that is, a beautiful story is always worth re-telling; second, a thing of beauty ‘moves away the pall from our dark spirits’, that is, it cheers us up. Things of beauty include the natural world (‘the sun, the moon, trees old and young’ etc.), and also ‘all lovely tales that we have heard or read’, and by implication these tales must include the myth of Endymion.

So if we take the ‘lovely tales’ to be the corpus of ancient myths, then ‘the mighty dead’ are the heroes of those myths, and the ‘dooms we have imagined’ for them are their tragic and heroic fates. The ‘endless fountain of immortal drink’ is a metaphor in which the myths are compared to a waterfall, and reading to drinking. The waterfall comes from ‘heaven’s brink’, meaning that the myths have a divine quality, and it is ‘endless’ because the stories can be re-told over and again, as Keats proposes to do in the remainder of the poem.

Answer 2

What is Keats saying in the last three lines?

Describing beauty, Keats says that beauty is also experienced in the grandeur and magnificence of the deaths of the mighty and powerful knights and kings (perhaps, though unlikely, any person who fights and did for a noble cause) who made supreme sacrifices and died noble deaths.

Magnificent tales of their lives are a perennial source of inspiration (like water falling from a fountain or waterfall) to the mortals on earth to emulate their lives and stories of their victories and heroic deeds.

Is Keats referring to "tales" of an afterlife / immortality in heaven? If so, why are those "tales" an "endless fountain"? Because there are so many different views on life after death?

These tales are like water falling from heaven (like a waterfall) onto the Earth bring forth a perennial source of inspiration for generations to come. I don't think there's any interpretation on “views of the afterlife“

And what might be the significance of the aquatic imagery ("fountain", "drink")?

I see no particular significance or inclination towards that but it might be an element Keats may have wanted to utilize to describe beauty.

Side Note: A Thing Of Beauty from Endymion is excessively focussed on in my school curriculum. You can look into a detailed study over here