Lynne Truss is very good on this sort of thing, try her book Eats shoots and leaves.
"The main place for putting a semicolon is between two related sentences where there is no conjunction such as "and" or "but", and where a comma would be ungrammatical." The semicolon tells you there is a close relationship, as in this example:
Tom locked himself in the shed. England lost to Argentina.
Tom locked himself in the shed; England lost to Argentina.
She also refers to your second usage of a semicolon, in list of items with extra defining words, as:
"It performs the role of a special policeman in the event of comma fights."
There are "annunciatory" colons: which proclaim the aproach of something.
There are "Yes!" colons which connect a qualifying phrase to the first part of the sentence.
"This much is clear, Watson: it was the baying of an enormous hound."
There are "Ah!" colons where the colon reminds us there is probably more to the initial statement than has met the eye:
"You can do it: and you will do it."
In your battle example I would tend to go for "the battle was over at last: everyone was dead." because the colon is classically used as a fulcrum. the twist in the tale makes the colon better because a semicolon likes the 2 halves to be thematically similar.
Anyway, read the book, she has a whole chapter on this
plus plenty more on all other aspects of punctuation.