A Power Failed to be Contained
A soul corrupted and stained
The Black Mancer will rise to the call
To defeat evil once and for all

If you’re going for clear and understandable, you’ve nailed it. If you were going for cryptic, it needs some work.

How can I make it more cryptic?

Just being nosey, but what is this for? Sounds interesting :)

@creativedragonbaby Don’t be so straightforward with your words. “A power failed to be contained” is rather self-explanatory. “A soul corrupted and stained”, in relation to the power—power corrupts. “A black Mancer will rise to the call” is again, quite self-explanatory, “to defeat evil once and for all”. It’s not that it’s not good, I enjoy it very much. So, start with the obvious and try some wordplay. The more tangents you create, the less people can trace your thought pattern. For example, the whole thing essentially says “someone who is evil and powerful will be defeated by a mancer”. It’s incredibly black and white. I want to stress that this is not a bad thing. If you want to be more cryptic, blur the lines. Make it so it’s not “good vs. evil” or one person defeating another.

You've got a wacky syllable count from one like to the next. 8 7 9 9.

8 syllables flow better in speech, but any even number tends to work. I think 8 or 10 tends to be the go-to for most people, but whatever works.

^ It can be whacky as long as it has a structure. For example, I change time signatures frequently when I play guitar, but it works because there is a mood change involved. It has something to latch onto. Same can go with writing.

For example, 8799...9978...8799...8799
Can be rewritten as:
87 9999 7 88 7 99 87 99
Notice the pattern.

My .02? It’s good. It would be just about perfect for a YA fantasy. If you are going for a more advanced audience it is still good- but I agree with others that it is a touch too much on the nose. Flavor to taste, but here is how I think of prophecy in fiction.
1. It must be relevant. Nothing sucks like stretching a prophecy to fit, or having a prophecy that could mean anything or is basically a map.
2. It tends to be best for there to be ambiguity as to wether it’s true or not. That not only leaves room for the reader and author to play with the idea of destiny, but leaves readers on their toes. Your whole story is right there in your prophecy. We know who the hero is when we see them, we know how it ends. The exceptions to this rule of course are if you either throw the prophecy out there and then it IS false, or you lead the reader. Like we meet A black mancer but he’s not the one- and later in the book/series “joe farm boy” becomes a black mancer. Other than that- you don’t...

... generally want a prophecy to give out all the story. Leading to...
3. Character ambiguity. Is the prophecy talking about this guy, or that guy? It doesn’t even have to be part of your story- simply leaving that door open gives the predictive reader another variable to chew on to help keep what’s coming a fun surprise. Many religious texts are actually good sources for exercise in prophecy. If you read some of their prophecies like revelations, and then try to logic out “what does that mean?” What actual objects, people, or events could the words be referring to? Read it non literally. You may not want to make your prophecy quite that cryptic- but it will get your mind thinking about the other ways you and say things in a less literal way.

^ Thanks for going more in-depth.

I will reword it, but I think I like it this obvious. It makes the reader think they know exactly what it's talking about, when it's the exact opposite. That kind of "Main character fills the prophecy, big whoop" is turned on its head

THE BLACK KNIGHT ALWAYS TRIUMPHS! https://www.youtube.com/watch?v=ZmInkxbvlCs

Did i get it right?

@guest_ Revelations is clearly about Nero. Once that realization kicks in the entire fucking Armageddon makes perfect sense. Nostrodamus, hell Negrodamus did it much better.

Why do white people love Wayne Brady?!
@creativedragonbaby my bad I'm a bit tipsy.
Think of the ravens.

Asimov would be the best though.