koj
異域の鬼(き)となる
外国で死ぬことをいう
also in 学国 and  学研故事ことわざ辞典 (as おに,となる)

異郷の鬼	20	9.8% (ment. in koj)
異域の鬼	185	90.2%

I think it's obvious in both the English and the Japanese from which perspective 異域/"foreign land" refers to, and I don't think that word alone makes this an idiom, but assuming it's mainly used in a non-literal sense then I think it's an idiom. It seems to be used both literally and figuratively based on koj, daijs etc.

@@ -20 +20 @@
-<gloss>dying outside of one's homeland</gloss>
+<gloss>person who died outside of their homeland</gloss>

In case my argument missed the mark (I might be confidently incorrect), I'll add that the usage of 鬼 here seems very non-literal to me. I think "becoming an oni" is usually interpreted to mean becoming fierce or wild rather than dying.

Google N-gram Corpus Counts
╭─ーーーーーーー─┬─────╮
│ 仕事の鬼になる │ 197 │ 🡠 becoming a workaholic
│ 復讐の鬼となる │ 224 │ 🡠 becoming a crazed with revenge
╰─ーーーーーーー─┴─────╯

Let's say Japan is the 「異域」.
Let's say I travel there and die, becoming a 「鬼」. Idiomatically, I am now also a 「異域の鬼」.
But if a Japanese person in Japan (the 異域) dies and becomes a 「鬼」, that person cannot be said to be an 「異域の鬼」, even though from my perspective the person is both in a foreign land (異域) and also dead (鬼).

I.e., a 「異域の鬼」 is not just anyone who dies in a foreign land. It is specifically someone who has traveled there from a foreign country. This relationship cannot be understood from the expression at face value. That's why it's an idiom.

I think the fact that the expression is found in multiple dictionaries of idiomatic yoji should serve as a hint.

@@ -18,0 +19 @@
+<misc>&id;</misc>

Daijisen says that 異域の鬼となる means "to die outside of one's homeland" - it doesn't make sense then to gloss this as "dying" rather than ghost/spirit
It's not an idiom because the meaning is literal

@@ -19 +18,0 @@
-<misc>&id;</misc>