Does Divine Word's Killing Effect Come Before or After the Banishing Effect (For Fiends)

Question

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.

Answer 1

The effects are simultaneous

On a failed save, a creature suffers an effect based on its current hit points

[...]

a celestial, an elemental, a fey, or a fiend that fails its save is forced back to its plane of origin

Both the hit point effects and the banishing effect are contingent on a failed save. The spell does not contain any temporal language ("after determining effects based on the creature's hit points", for example). Therefore, both effects happen simultaneously.

Is the fiend killed here or in its home plane?

Xanathar's Guide to Everything provides this optional rule for adjudicating simultaneous effects:

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen.

If the cleric is a PC and is casting divine word on their turn¹, then XGtE allows the cleric's player to decide the order.

If the DM doesn't use the XGtE rule, the order is undetermined. Determining the order would therefore be up to the DM.

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¹ Divine word is a Bonus Action, so it's difficult to cast it outside of your turn. Readying wish would work.

Answer 2

Generally, it makes sense to apply a spell's effects in the order they are listed in the spell's description, unless otherwise specified by the spell.

Divine Word says:

You utter a divine word, imbued with the power that shaped the world at the dawn of creation. Choose any number of creatures you can see within range. Each creature that can hear you must make a Charisma saving throw. On a failed save, a creature suffers an effect based on its current hit points:

50 hit points or fewer: deafened for 1 minute

40 hit points or fewer: deafened and blinded for 10 minutes

30 hit points or fewer: blinded, deafened, and stunned for 1 hour

20 hit points or fewer: killed instantly

Regardless of its current hit points, a celestial, an elemental, a fey, or a fiend that fails its save is forced back to its plane of origin (if it isn’t there already) and can’t return to your current plane for 24 hours by any means short of a wish spell.

If Divine Word affects a fiend with 20 or fewer HP, it should be immediately slain, then banished to its home plane (assuming that slaying it doesn't do that already, like it would for a summoned creature from a spell like Conjure Fiend). The other effects of the spell would be applied in order, but because they are redundant, you can usually just skip to the bottom-most effect. If there were a creature that had the ability to come back to life after being slain, tracking the other effects would be necessary, however, as the creature would still be affected by them until the duration expired, since dying wouldn't technically remove them.

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Alternative Interpretation: The effects happen simultaneously

In the event that multiple effects happen simultaneously, Xanathar's Guide to Everything includes the optional rule that the controller of the creature suffering the effect chooses the order in which they are applied. So in this case, since the DM controls the target, they would choose whether the creature is slain or banished first.