Treat it like it is written
Your magic and an offering put you in contact with a god or a god's servants. You ask a single question concerning a specific goal, event, or activity to occur within 7 days. The GM offers a truthful reply. The reply might be a short phrase, a cryptic rhyme, or an omen.
This is the text of the spell. Your reply needs to be truthful, but it doesn't need to be straightforward or easy to decipher. It requires the players to ask the right question. Also, they need to ask about the future, they can't ask about the past with it. And don't straight out answer yes/no. So if they try to reality-hack, the gods will answer accordingly. Let's call this way "type 1" and see examples below.
The spell doesn't take into account any possible circumstances that might change the outcome, such as the casting of additional spells or the loss or gain of a companion.
The spell also doesn't figure out what you asking the question does change. It also doesn't even try to figure out if acting differently might change something. That's "type 2", and again, examples are below.
If you cast the spell two or more times before finishing your next long rest, there is a cumulative 25 percent chance for each casting after the first that you get a random reading. The GM makes this roll in secret.
You get one long rest a day. First spell: 0% fail. 2nd is 25% you get "the sky is blue" as an answer. 3rd: 50%, 4th: 75% and if you roll the 5th, don't even bother asking, the GM should answer akin to "There will be rain with sunshine in a town 400 miles east... stop calling already."
Type 1 - Message received, Information unclear
The answer creates the plot, as it does give imperfect information!
Polybus: "I ask the gods who will be responsible for my death."
Gods: "Your son will become the one that will bring the death of you."
[Polybus his baby son tossed to animals, but he ends up as the adopted son 2 kingdoms over. A dozen and half years later]
Oedipus: "What does my future bring?"
Gods: "You will kill your father"
This is a classic from Oedipus Rex. Both answers are absolutely true, and while the end result of the first technically is outside of the 7-day span of the spell, the very act of putting Oedipus on the track to kill his father happened just hours later: by having Oedipus sent away, he ends up adopted at the other court not knowing who his real father is - and when he flees his adopted father to try to prevent killing him, he promptly meets Polybus and slays him, his real father.
But the first prophecy even would have held true if Polybus doesn't send Oedipus away: Young Oedipus is praised for being the most beautiful child and that sows the seed of discord in a neighboring queen, who sends people to kill the child on his first birthday - and Polybus saves his son, dying in the process. In another alternative, the toddler distracts Polybus at the wrong moment during a court session and an assassin strikes down Polybus within that week. And in yet another alternate line, by loving his son too much and not casting Oedipus out but instead sheltering him more than ever (and too much), the young boy contracts a contagion and brings it home years later.
The main reliance here is: The first answer only sets something in motion that, even if avoiding a death within 7 days, at some point much later will result in death. However, the event that set things in motion happens then.
Both prophecies rely on information the asker doesn't have: The first prophecy never inquires about the method of the death, or when it will occur. The second lacks the identity of the real father.
Double Meanings: Damned if you do, damned if you don't.
Athenians: "How do we defend Athens?"
Gods: "Build Wooden Walls."
Wooden walls... We know that these are ships and the battle of Salamis gives us that knowledge in hindsight. But at the time, it could also be read as building palisades and barricades, turning Athens into a fortress city with its Akropolis impenetrable.
Don't tell them, but show them an omen!
Thebans: "Will the gods bless the harvest if we sow tomorrow?"
Gods: An Eagle catches an early Owl over the Nile.
What do the gods want to tell our inhabitants of Theben, Egypt? Is it the Eagle, a bird seen as honorable and godly and a good omen in general, or is it the Owl, which is seen as an ill omen as it hides during the day and flies silently? Shall the people sow tomorrow or shall they be on the lookout for the hidden danger instead?
Sometimes, the answer is just the icing.
Players: "What does the road to Baldur's Gate hold?"
GM: "The gods show you a flyby of the road to the town, and it is a long and windy one. You spot about 4 other settlements before you see the gates to the town and they open."
If you played Baldur's Gate, the original, then that would be the prophecy someone might get at the start of the campaign: you don't get to be in the town till the midpoint of the campaign. But the answer glosses over all the fights or dungeons that are also there.
Other times: the Gods are Jerks
Players: "Is the Road to Baldur's Gate safe?"
GM: "More dangerous than your halfling hill home and safer than the road from Easthaven to Kuldahar."
Yup, the gods can be jerks giving totally irrelevant comparisons. For the record: the halfling hill home is Bag-end, which at the start of the Tolkien Saga is like the safest and most peaceful place in all the world, and the road from Easthaven to Kuldahar is a reference to Icewind Dale: When the noobie adventures take it at the start of the campaign, their travel group is assaulted by frost giants and decimated to just the party. So in other words: the gods say "you will face some danger, but not what.
Type 2 - Or: Fate does not expect meddlers.
If you do nothing unexpected...
Player: Where does the Overlord put his scepter on the night of the 14th to the 15th?"
GM: "[...] he'll put it onto his nightstand, chained to his four-headed guard chimera."
Congratulations to the player that was fairly specific at the act he's asking about! Though... Do you see the implied [...] up there? that's where the five magical words "If you do nothing unexpected" belong. Unexpected can be a lot: Do the players raise a ruckus to divert attention, to a different place? Well, Fate did not expect that, and now the Overlord has his scepter forgotten on the dark cathedral's altar, as he is out of the house turning one of the PCs into a statue for his lawn.
Preparation is dangerous!
Player: "In what town will the Lord be on the evening of 14th?"
GM: "Lunden."
Player: "Ok, we travel to Lunden, and on the evening of the 14th, I cast..."
Ok, it's a nice idea to try to pin down the location of the target in advance. And once the area is clear, and a little legwork can help there, Locate Object (2nd) and Locate Creature (4th) come to play, your standard homing beacon spells to try and find the exact location... Till you reread that this is Type 2. By casting the Locate X spell, you actively trigger the second stance: you might change the outcome.
The locate spell might trigger a detection spell, like Detect Magic (1st), or it runs into some item that triggers when a locate or scrying spell targets the area - in any way, the pinpointing triggers alarms and the Lord instantly leaves the town, leaving behind a trap. Oh, and as icing: the prophecy was true till that moment...
