ここまで literally means "up to here/this place". ここ -- similar to the English word
here -- does not always literally refer to a place and may be used in an extended sense.
"I don't like anonymous downvoters. So here's my opportunity to say something about it: 'Please leave a comment if you downvote this post.'"
here(3): used when indicating a time, point, or situation that has arrived or is happening -- Oxford Dictionary of English
As for your sentence, we can analyze the first part in isolation.
In this sentence, we could say that ここまで effectively modifies 乱暴. At least, we don't need the rest of the sentence.
ここまで乱暴とはな would already be a valid statement.
ここ, instead of a point in space, points to a point on the 乱暴-scale. まで marks it as an upper limit (or rather extent) with a negative (or positive, depending on the polarity of what it modifies, see below) connotation, ie. that it is rather high up on the scale. Which may be good or bad.
Normally, 乱暴 carries a negative connotation, but if said by some street gangster, it could be positive. Or perhaps easier to understand, it can be used with a positive attribute:
This doesn't work, at least not with the same interpretation as above, where ここまで modified 乱暴. There is no adjective or anything else where it would make sense to modify it. You can't modify そんな (=such a), which is a modifier for こと already. The phrase after ここまで is
そんなことは初めてだ. It doesn't make sense to modify this either, as you would end up with something like
the extent, to which such a thing is being the first, is high.
However, the sentence is grammatical under another interpretation; that there had been no such thing up until now.
ここ usually marks the current place (here), and here, in extension, the current time (now). So if you're confused with the usage of ここ here, perhaps you could think of it as
up to here and now. Another rather common sentence pattern is:
(ここ・今)まで そんなことは なかった
(Up) until now, (such a thing/that) didn't happen.
Language is complex and given the right context, a lot is possible. Here's one more sentence I found:
(The beginning of a review of a game from a series. Based upon past games, the reviewer expects something that's normal average and simple to play.]
だけどここまでそんな気持ちを裏切られたのも久しぶりのような気がする (It feels it has been a while since that feeling/expectation/mood got betrayed this much.)
It's just coincidence that そんな occurs directly after ここまで -- it just specifies 気持ち, namely what the author mentioned in the previous sentence.
But betraying (そんな気持ちを裏切る) is an action which may be modified by specifying a degree, eg with ここまで, and that's what's happening in this sentence.
As for your sentence,
it's the first time something like that happened, you could just say:
Optionally, you can make it more specific and describe how much of what it is:
- 大それた wild, crazy, mad, outrageous
- 妄想じみた foolish, delusional
- 常識はずれ far beyond common sense
- 前代未聞 unheard of, unprecedented
These sentences are of the pattern:
(ここ・そこ・あそこ)(まで) (adjectival phrase+こと)は (some statement)
Literally, you could interpret it as
A thing that is X to such an extent that [statement].
Finally, some real and more interesting examples from real life:
＊The choice of the right word from the こそあ(ど) series depends upon the context and your relation to the matter at hand.
- (伊豆屋旅館の口コミ情報, review by a visitor) 混浴ブームとはいえ、まさかここまで身も蓋もない混浴があるとは。
In the first sentence, the author is somebody who had first-hand experience. In the second sentence, the author refers to a 補足 from a post above (そこ). In the third sentence, the author refers to a talk or comment by somebody, the relation already being specified by あの立場.