I'd imagine making a custom status effect in itself would be difficult enough, but having it affect the player is even harder. I'd personally wait until they add status effects on players in the future, but if you seriously need it quick then try what you can. I'll be glad to help.
I'm not looking to code anything into the engine just yet - just asking whether it is possible to use the existing XML/Behaviors to add an immobilizing effect like the Ranger's ability does on mobs. I'm guessing it's a no but I still want to be sure. The abilities for casters are kinda limited at this point. The ones I can name are:
teleporting, casting projectiles(homing or non), summoning, nova, and healing.
Don't get me wrong these are all usable and fine - but still quite limited. I'm trying to make mobs that don't behave exactly how mobs behave in vanilla hw
There are plenty of abilities that bosses use that don't seem to be available for use by other mobs as well which is a bit limiting.