Single word for emphasising 'very good' or 'very bad'

It is first apparent that "the extent that you cannot believe" is not idiomatic/correct and rather you'd have to say "to an extent that you cannot believe" or "to such an extent that you couldn't believe it". "The" being a definite article you imply when you use it in this context that Joseph is aware of a given degree or extent of wickedness that he doesn't believe; this possibility is highly unlikely.

You can use a shorter phrase such as those
"wicked beyond belief" (lit.),
"wicked beyond (the common) measure" (lit.),
"wicked beyond your belief" (user LPH).

Others from the literature

wicked beyond compare
wicked beyond all comparison
wicked beyond the description of human words
wicked beyond the power of words to express
wicked beyond any mortal reckoning
wicked beyond imagining
wicked beyond description

I am looking for a word which can be used in emphasising that [no quotation mark] something is very bad or good to the extent that someone else cannot believe. [no quotation mark]

For example: I may tell Joseph that: "Joseph! To an extent that you cannot believe, Sarah is wicked" or "Sarah is wicked to an extent that you cannot believe".

Is there any other word or group of words I can use to explain to Joseph how wicked Sarah is without using the phrases "an extent that you cannot believe"?

Your phrasing is not natural. This is more conversational: Sarah is unbelievably wicked.

The phrases the extent and an extent are used differently. Here, an is correct.

Only use quotation marks to enclose quotes or titles.