A magnetic monopole is a single, isolated magnetic pole: A North without the corresponding South, or vice versa.
Electric monopoles (electric charges) are well-known and ubiquitous. Magnetic monopoles (MM), on the contrary, have never been observed: if you break a magnet in two, the two halves will still have a North and South pole each. This remains true even when the division reaches the subatomic level (indeed, the magnetism of macroscopic magnetic objects is caused by the spin of electrons which are themselves analogous to magnetic dipoles).
For long time, physicists have thought that MMs could not exist in nature as their existence would violate Gauss' Law, which is one of Maxwell's equations and the law of conservation of energy for Electromagnetism.
In the first half of the 20th century, however, physicist Paul Dirac theorized that MMs should exist, based on symmetry arguments and on the nascent Quantum theory. Nowadays, many current theories in high energy Physics predict or imply the existence of MMs, and research is ongoing on ways to create or find one.
According to some theories, the reason why we have never observed a MM is because their creation requires a great deal of energy, much more than we are currently able to produce in particle accelerators and in the same order of magnitude as the energy density in the early universe. So, only a few were produced before the universe cooled down; inflationary expansion intervening since these times means that those few are far between. And I mean faaaar.
What does all this have to do with your story?
Your future society could have found a way to artificially produce magnetic monopoles and use them to power Alcubierre drives (I know...I'll get to it).
Now it just so happens that the planet they crash-landed on is where one of the feeew naturally occurring magnetic monopoles (the original ones created when the universe was still young) is buried.
Disclaimer I'm not a high-energy physicist so I have no idea if a magnetic monopole could sit happily in the middle of an habitable planet in an ordinary star system, but
- planets can be electrically charged
- we don't know much about magnetic monopoles anyway
so I think you could pretend that it does.
It could also be that this (the fact that the planet houses a magnetic monopole) is the reason why their monopole-powered ship malfunctioned and crash-landed.
I bet all their instrument were giving out crazy readings in proximity of the monopole but they are more likely to have thought of another disturbance/breakdown than of a naturally occurring MM. It's likely that, since they can create them artificially and the natural ones are so rare, humans have altogether forgotten that they may occur naturally and thus wouldn't even think that they have came across one.
The biggest problem in this idea
or, something for you to write your way out of:
- magnetic monopoles do not mean energy for free.
If MMs do exist, this means that there is complete symmetry between electrical and magnetical forces. So you can't get endless energy out of a magnetic monopole any more than you can get it from electrical charges.
So what remains for you to explain or handwave is how magnetic monopoles can power an Alcubierre drive and how it is related to magic powers of biological creatures.
Thinking about it, I am not sure that this answer solves more of a problem than it creates :D