Nearly every epic adventure has one or some of these things. Not everyone is a poet, and sometimes it can be hard to write the prophecy or riddle the party needs. Even I have trouble when I sit down to write out a prophecy, or poem. There are some general pointers that are important to keep in mind no matter what.
It doesn’t have to rhyme. Maybe you can’t find the words you want to say that work in a rhyme. Maybe the original was in an ancient language, or elven or draconic. When translated to common it might not rhyme any more.
Here is an Italian lullaby called Boil Boil little pot. As you can see it rhymes easily.
Bolli bolli pentolino,
fa la pappa al mio bambino;
la rimescola la mamma
mentre il bimbo fa la nanna;
fa la nanna gioia mia
o la pappa scappa via.
but when put into English…
Boil, boil, little pot,
Cook the food for my baby.
Mommy mixes it
While the baby sleeps.
Go to sleep, my joy,
Or your food will run away”
Use the thesaurus. The words need to stand out, you need to do some real word smithing to make sure the writing isn’t just mundane. In the same breath don’t glitz it up to the point that it is just pompous gibberish. As a DM i’m sure you know that words can be used to powerful effect. Words can evoke emotions, and that is what your writing needs to do.
Be vague… Don’t list out exactly what they are supposed to do. Leave it wide open for them to interpret. You might mean one thing, and they players may interpret it differently. If they do, and it goes with the story, run with it.
Maybe it wasn’t all written by the same person. This will allow you to change the tone. Perhaps a monastery of monks have been having visions and adding to the prophecy for centuries.
One of my favorite examples is the Karaethon Cycle from the wheel of time.
Whatever you choose to do make it fun!