Emphatic adverb often used before adjectives or descriptive phrases to mean 'extremely' or 'truly'. Common in literary or expressive contexts.
It was an extremely beautiful view.
It was an extremely strange occurrence.
I heard a very strange story.
She sang with an extraordinarily beautiful voice.
実に and 世にも both add emphatic, somewhat literary force to a description, but 実に is more common and can stand before a wider range of words.
Compound of 世 (yo, 'world') + に (ni, particle) + も (mo, 'even'), literally 'even in the world'. The exact historical derivation of the intensifying sense is uncertain, but it likely developed from a rhetorical expression meaning 'in the whole world'.