The fighter's friend is hurt, even dying. The fighter prays to the gods to heal their friend. How is this action resolved? If it's a Religion or similar sort of check, what's the DC?
I think this is something that can happen in D&D.
In 5E, I'd ask for a DC 20 Intelligence (Religion) check. On a success, an intermediary of the gods delivers the requested aid. On a failure, something else happens.
I wanted to expand on some of my thoughts that led to this response.
As I said, I think it can happen, by which I mean I think the action declaration fits within the genre of D&D as I would imagine it's played usually. Though I wondered at first if the fighter might not have the requisite fictional positioning to have a chance of having his prayer answered. After all, I thought, wouldn't it depend on whether the gods were present? This was because in my mind I was treating the gods as NPCs whose presence would have to be established before any attempt to persuade them could be made, and, at this point, I was imagining that the attempt might be resolved with a Charisma (Persuasion) check. I mean, the fighter certainly has the fictional positioning to say a prayer, but is anyone listening?
Now, it occurred to me the player, by making this action declaration, is raising a proposition about the nature of the gods, that they are present and responsive to prayer, but all that really needs to be resolved is whether the fighter's friend is healed or not, so this question about the role of gods in the setting can be folded into the check. It doesn’t require a separate resolution. Again, the fighter certainly has the fictional positioning to voice a prayer in the hope it is answered.
So I would want to bring this to a check, rather than just saying
yes, because there seems to be a lot at stake in this situation. At first, I was thinking of calling for a Charisma (Persuasion) check because it looked like a social interaction, but praying to the gods might not depend on what you say or how you say it. It really depends more on the nature of the gods themselves, which brings me back to the player's proposition. The player is proposing a fiction where
the gods are such in this setting that they will grant the fighter his prayer. Now, because I try to play with little or no "myth", I will generally call for an Intelligence check when an action declaration puts something at stake regarding some detail of the setting that has not been established. This favors characters with high Intelligence and/or proficiency with knowledge skills to be the "lore-masters" from whom the table learns new things about the setting giving the player some authority over that aspect of the setting. I think lore about the gods falls under the broad category of the Religion skill, so I'd call for an Intelligence (Religion) check from the fighter to resolve whether or not the healing is received.
To set the DC for the check, I often look to the 1E DMG when I'm called on as DM to provide adversity to the PCs because it contains a wealth of rulings regarding genre appropriate content introduction, and page 112 has a fairly straightforward system for determining what difficulty an appeal to the gods might have. When converting a percent chance like this into a DC for a 5E d20 roll, I assume an average PC ability score modifier of +1, so the straight 10% chance of intervention in a case where the PC has received no previous intervention from the gods, has exhibited exemplary alignment behavior (whatever that means), the intervention will not bring the gods into direct confrontation with other gods, and it hasn't already been established that the PC is closely serving the gods in some way, would be converted into a DC 20 check. If it has been established that the PC is serving the gods closely (operating either under direct instruction or through an intermediary), the base DC would drop to 15. Each previous intervention received would raise the DC by 1 point. Deviations from exemplary alignment behavior would raise the DC by 1 or 2 points, depending on the severity. If direct confrontation with another god or gods would result, the DC would go up 2 points. In any case, if the player rolls a natural 20 on the check, I would give a 20% chance of an additional percent chance equal to the PC's level that the specific deity or deities called upon intervene directly.
Consequences for failing the check would depend on previously established fiction and would range from the simple opportunity cost of using a turn and risk of a failed death save if attempted in combat to other more interesting consequences if attempted outside of combat including the possibility of the prayer being answered by an unintended deity/powerful being.