What is the adverb of cruel?

adverbs can describe verbs adjectives, and other adverbs. A dictionary would suggest that you use "cruelly" instead of "cruel" as an adverb:

"he sneered cruelly"

I can't think of a situation where "cruel" would be the adverb. A good test for adverbs is that:

1) They often have a (-y) suffix

2) They still make grammatical sense almost anywhere in the sentence:

"cruelly, he sneered" ; "he cruelly sneered"; "he sneered cruelly" .