Is it really only described as an adjective? And if so, in which dictionary? Because at the very least it should also be a verb - to astonish. Even Internet Explorer spellchecker knows that.
And it it's a very, then it can be a noun, just like those others. The only reason they already appear as nouns in the dictionary is that they are already used in that context.
"Astonish" is a seperate word, or rather, seperate entry in the dictionary. And yes, it's a verb. A verb can be turned into an adjective or a noun by putting a suffix on the verb like "-al" or "-ence" or "-ing" or several others (to deny - denial, to depart - departure) etc. Not all verbs are turned into nouns, and not all verbs are turned into adjectives, even though you theoretically could.
Look at the difference:
https://dictionary.reference.com/browse/happening?s=thttps://dictionary.reference.com/browse/astonishing?s=tWhereas "The Happening" uses -ing in the following manner: "a suffix of nouns formed from verbs, expressing the action of the verb or its result, product, material, etc. ( the art of building; a new building; cotton wadding).", "Astonishing" uses the -ing in this manner: "a suffix forming the present participle of verbs ( walking; thinking), such participles being often used as participial adjectives: warring factions."
While you can create whichever words you want in your language, it's the common usage that decides how it is interpreted. "The Astonishing" is not in common use. It's not inherently understood what it even means. "The Happening" IS in common use, and people knows that it refers to an event. "The Happening" is not commonly referred to as an adjective either, so there's no potential double meaning. When people come across "The Astonishing"; it's almost exclusively an adjective. It's jarring, just as "The thinking" would be jarring, or "The walking", or "the running" - and yet, all three of THOSE examples DO have a defintion for the noun counterpart of the word. "The astonishing" is more akin to "The amazing" - it has no established use as a noun, and in fact, you would not say "the amazing" even if you can, you would say "the amazement".