The quality or state of being furious; extreme anger, wild passion, or violent intensity.
From furious + standard English suffix '-ness' (quality of state). This is the standard, modern way to nominalize 'furious' in contemporary English.
The '-ness' suffix is English's most reliable tool for turning adjectives into nouns, and 'furiousness' is the proper way to express this idea, though 'fury' itself is shorter and more poetic.
Complete word intelligence in one call. Free tier — 50 lookups/day.