Both my kids had terrible night terrors for years, then it stopped completely when they got older (7 or 8 yrs old, I think). I believe it is a combination of an immature brain (normal for their age at the time) and sleep deprivation.
Things that I discovered that may help:
1) Daytime naps. Oddly, lack of sleep can bring on night terrors.
2) Early bedtime. The later they went to sleep at night, the more likely it was that they'd get a night terror.
3) Mineral suppliments in small ammounts. Calcium and magnesium suppliments for kids, especially magnesium. (I wish I knew this sooner).
4) If out of diapers, make sure they go to the bathroom before bed. If they had to pee in the middle of the night, instead of waking up fully, they would have a night terror first.
For us, this went on for a long, long time. For some people, it passes quickly. If it concerns you, I'd go see a doctor who knows something about toddler brain development, like a pediatric neurologist. Night terrors can be nothing at all and will pass. Or can be sign of other issues.
I found there was no use whatsoever in trying to comfort them. Only makes it worse. Hard to see them so terrorized and not react, but there is really nothing that helps once it starts.
Best wishes for a good night sleep for all concerned.