extremely vs deathly

extremely is not an adjective while deathly is an adjective.

Word	Noun	Adjective	Verb	Adverb
extremely	No	No	No	Yes
deathly	No	Yes	No	Yes

As adverbs, extremely and deathly are synonyms defined as:

Other synonyms of extremely include exceedingly, super.

extremely (adjective)	deathly (adjective)
	having the physical appearance of death
	causing or capable of causing death

extremely (adverb)	deathly (adverb)
to an extreme degree	to a degree resembling death
to a high degree or extent; favorably or with much respect	to an extreme degree