Solution:Legal (Adjective) legitimate; law abiding. =Illegal (Adjective) = unlawful; illegitimate.
Threat (Noun) = something that is a source of danger; menance.
Extinction (Noun) = no longer active; no longer in existence; extermination.
Faunal (Adjective) = the animals of a particular region or period.
Hence, illegal is the antonym of legal.