This spell apears to have been nerfed. It used to be 'Cast As Required' now it's Cast at the start of a characters turn. But IF cast on a creature you can evade it....This makes it annoying how do you play it?
You would really like to land on the plains, 3 squares away that has Solomons Crown on it, but it's guarded by a dragon. Do you have to cast the spell on the dragon before you roll, hoping that you get a 3, and can therfore evade him. If you roll a 4, you have just wasted your spell and given the opportunity for somebody else to go there and evade the Immobolised Dragon.
Or can you roll to see if you get the 3, and then cast in on the dragon, before you actually move. If you roll a 4, you save your spell and don't waste it.
Old School rules you could land on the dragon and then cast it. In fact of a dragon jumped out on you, and you fought it and lost, we'd always play that you could still immobolise it, symbolising your spell casting freezing the creature in it's death blow.
But with all this new nerfing going on, it's hard to keep up.
I talk nerfing as I'm used to 2nd edition and playing many years ago, the new 4th edition I believe still had this, before the Upgraded FFG version nerfed it and made quite a number of things... lame.