Exclamation point inside a sentence

Solution 1:

Answering per OP's request:

This is an antiquated style of punctuation, seen primarily in pulp fiction of the late 19th and early 20th centuries. It is a kind of parenthetical intensifier. Or you could call it an inline aside. Nowadays, such a thing would probably be rendered in parentheses complete — "... and (alas!) the very diablerie of the woman" — or with just the exclamation mark in parentheses: "... and alas (!) the very ...." etc.

Solution 2:

It's just to clarify that the term (alas, in this case) should be stressed.

Other ways to achieve the same result are:

• (alas!)
• alas (!)

Here's another example:

The huge crowd (1337 persons!) gathering in my front yard wasn't really that friendly looking.

Here, the "(1337 persons!)" stress the fact that it was indeed a huge crowd to be gathering in the front yard.

For the history of this usage, see the first comment on the original question.