In your example, the principle that determines whether to capitalize wild terrain should be whether a name or term is unique and perceived as a proper noun by the writer. There could be many wild terrains in anywhere in the word. We can't capitalize it just because the terrain is wild.
If you contrast wild terrain with Death Valley, you could notice that Death Valley is capitalized even though there could be many death valleys that are extremely dangerous to people anywhere in the world. Death Valley is capitalized because it is a proper noun conceived by many people as such and it indicates only one death valley located in eastern California.
If wild terrain could be perceived as a proper noun by many people, it should be capitalized like Death Valley. Until then, it is appropriate not to capitalize it. The linked Wikipedia article on Capitalization explains it as follows:
The capitalization of geographic terms in English text generally
depends on whether the author perceives the term as a proper noun, in
which case it is capitalized, or as a combination of an established
proper noun with a normal adjective or noun, in which case the latter
are not capitalized. There are no universally agreed lists of English
geographic terms which are considered as proper nouns...
Conclusion: It depends.