What happens when a boulder ends its movement on a familiar? Is it stuck until the next time the boulder moves?

Sadly I remember and always considered familiars indistructable forces of nature...since nothing can hurt or sunder theire movement. (mud and terrain, might stil work, but they still can run trough monsters.)

Well, familiars can be halted by doors, boulders and walls (moving or not), and according to RAW even by stairs, since they are not allowed to perform movement actions (though I don´t play it that way).

So, it´s clear that a familiar cannot move underneath a boulder. If the boulder ended its movement on the familiar, then it obviously cannot move onto another space covered by the boulder.

Whether a familiar can move out from under the boulder is unclear. The closest reference I can see is in WoD Quest 1, where a familiar is replaced on the next tile if the one it was on just collapsed. So it basically was moved "out of the wall" by the quest rules.

I guess I would allow it to move clear with the restrictions mentioned above.

I suppose it comes down to a house rule, but I think familiars should have to stay put until the boulder is no longer on top of them. Further, if Boggs is under the boulder, he can't prevent spawning. Again, just my interpretation.