I've rescued Haer'Dalis and I followed him and his band of troupes into the portal to the prison and the Master of Thralls is mopping the floor with me he keeps casting hold on me and then he begins to rape all my character's what can I do? I don't no what I can cast to prevent this

You have several options, some of them are:
1) Have a mage out of the Master of Thrall's sight and cast Dispel Magic (level 3 mage)
2) If you have a paladin and the Holy Avenger, wack your party member who are held with it since Holy Avenger dispells magic when hits target
3) Cast Free Action (lvl 3 cleric) from your priest, provided he/she is not held, you can also cast this before battle I think
4) Ring of Free Action
There are other ways, the above are just some of them.

For #1 above, provided your mage is not held and Dispel Magic is cast on your held party members.

I brought in some undead (who are immune to most forms of mind manipulation) and had them go after the mage. Clerics can bring in skeletons at relatively low levels (I was using a Ranger though whom I class changed to a Cleric). I was able to cast the spell twice and had them initiate contact. The spell of free action is a good spell too as anything that will affect movement just doesn't work on the character. If you can bring in any other monsters, this will help you overwhelm him too. Just try to take him out as quick as possible. Any other characters that are good with bows will help keep him from casting too. Never underestimate magic missile too.

minor spoiler

I beat that scenario first try and by SHEER LUCK, I think! But try using many summoned creatures that will draw his attention. Then use whatever magic piercing/reduction spells you have to weaken him and fire all the other offensive spells. I know I didn't have Holy Avenger yet, but the cleric spells (free will etc) helped as did potions for same thing. I rested party before going after him and fought to the bitter end, so as not to have to start all over again. I think the Harp of Dischord (if the name is right, that's the one you should have gotten when you freed Haer'Dalis) actually works on everything you encounter in the prison (topside). Love that harp!


Cloudbringer

I had the ring of air control, charmed one of his air elementals with it.
It immediately ripped into him preventing him casting - while my party ducked back out of the way luring the other air elemental & finishing him off by himself.
By the time the first elemental 'went red' again, it was fairly low on health, and the Master of Thralls was Near Death, so we finished them off without taking much damage. I think if I hadn't had that ring it would have been a completely different story though ....