My son has just got over a stage of night terrors - he was approx 12 months when they started.
Same story - eyes wide open, screaming, thrashing, didn't want to be held/put down... Nothing calmed him down. It took about 4 hours to get him to stop screaming on several occasions.
I spoke to the HV who said they're quite common and there unfortunately isn't much you can do do stop them. Apparently they usually coincide with a period of developmental change. My son started walking on his 1st birthday and that was pretty much exactly when they started. I just hadn't put 2 and 2 together.
Anyway - my advice...
We did as some other people have suggested - bring him downstairs, telly on, lights on because it was the only way he would stop freaking out. It worked and finally he would be calm enough (and awake by that stage, I think) to go back to sleep).
After three weeks of 2-4 hours stints trying to calm him down - I decided not to bring him out of his room anymore, but try and calm him down in his nursery. I kept a store of books in his nursery (he loves them so they were a great help to stop the sobbing) and used to read him books over and over again in the dimmed lights of his nursery.
It then started to take only 20 mins to calm him down. Hooray!
The problem we then encountered was that although he was calm and quiet - he was used to playing in the living room with Mummy and Daddy until he felt tired again - so we had to do another period of sleep training to get him back to understanding that it was night-time, not play time. And that he'd have to cope with just one of us.
Finally he went through a short period of waking up grizzling; a far cry from the terrifying night screams we'd experienced and he'd very quickly put himself back to sleep.
Last point - I also cut his sugar intake right back at the same time. We'd crept into giving him bits of icecream, cake, anything nice we were eating and I truly believe it had an effect. After we stopped the sugar things quickly returned back to normal. About a month ago my MIL gave him some cake and I remember saying 'if we're up at 3am I'm going to phone you up and tell you!' - and bugger me... we were. We had three more nights of terrors. So now - I just stick to 'baby/children' desserts with minimal sugar and he seems much much better.
The thing to remember is - it feels like the end of the world while they're happening but the whole episode was over in about 6 weeks. And it's quickly forgotten.
I'm sure I'll regret saying this but he's been sleeping through 11-12 hours every night since we got it sorted, so it doesn't affect them forever. Although I'm sure he'll have another episode at some stage or another.
Try not to listen to some people who tell you its because they're starting to remember things from the daytime in their dreams, or whatever - I used to get so upset wondering why his life was so bad suddenly that he would wake up in such a state. I truly believe it has no relevance with conscious behaviour. DS1 is a sunny, happy child normally but the 'night terror' child was like something out of 'The Exorcist'.
Sorry for the massive post - but I hope some of it helps. Best of luck.