I've had repeated cysts although none have been dermoid cysts. Ask relatives and see if it runs in your family. I just found out that my mother also has fibroid tumors for years.
My gyn told me that once you have a cyst (dermoid, hemmhoragic, or endometrial) you have a better chance of getting another. But she did also tell me that it's about the same chance as getting the initial one in the first place. So I guess you're more prone to them just because you got the first one.
Mine is much smaller than yours, but the symptoms are really annoying and starting to get kind of debilitating. I can't imagine what a 25cm cyst is doing to you.
Hopefully they will get your 'alien' out in one piece and be able to leave ovary. I was told that if my ovary was damaged from the cyst that they would likely take it out too.
But apparently you can still get them on the other ovary just as easily. (I know, that's so reassuring)
But happily, as I'm sure Tascha will be around to tell you too, 99% of cysts are benign and just a major annoyance. :)