There may be a magnetic field, but it obviously proves no use for the inhabitants of Yliakum. They would have no concept of north and south. In my opinion, it's also counter-intuitive for players to hear "north" and the other cardinal directions when they do not exist in-game. Why not create a directional system based on three dimensions?
The first dimension is distance from the Crystal. Measured in a straight line aiming down to the center of the lowest level. It might be more sensible to measure from a certain point on the ceiling of the stalactite, then treat it like a meniscus, since it would be difficult to measure the Crystal's exact dimensions. (I'm not sure if scientists in Yliakum are capable of this). Of course, most people would never need to use this dimension in their daily lives. Engineers would find it useful, however.
The second dimension is a degree. From 0 to 359, then back to 0. The zero degree should probably originate at a point of importance. The degrees increase in a clockwise direction. (This would make the most sense, I think.)
The third dimension is distance from a point in the line mentioned in the first dimension closest to you. This is how far you are to the edge of the level you are currently standing on.
For example: 520 km down, 107 degrees, 44 km out.
If you were 2 km down, 298 degrees, and at least several km out, you'd be climbing somewhere on the walls of Yliakum, very close to the ceiling.
If you were 700 km down, any number of degrees, and 0 km out, you'd be falling to your death in the ocean below.
If you were 0 km down, any number of degrees, and 0 km out, you'd be dead. (Inside the Crystal.)
Where this method fails is you cannot tell someone to simply go east or west; you'd have to give them a specific location, at least detailing the degrees and how many miles/km outward. But given the fact that Yliakum has no compasses, this is how they would manage, I believe. There would probably be a heavy reliance on maps, the shadows cast on objects by the Crystal, and general knowledge of landmarks. People who travel often would memorize the degrees and distance outward for many locations. Of course, players could probably use a minimap to speed things up.