This question is stated right in the title.
In this case, a cleric might cast Divine Word with a fiend with 20 or less HP in the area, and the fiend can hear the cleric.
As we all know, if a fiend fails its save against Divine Word is banished to its home plane. However, all creatures with 20 HP or less is killed instantly. Would, say, a demon that failed its save while under 20 HP be killed first, or sent to the Abyss, then killed there? Same thing with devils, night hags, rakshasas, yugoloths, et cetera. The yugoloth home plane is Gehenna, for specifics.