Info on Serotonin Syndrome?

I've recently had friends invite me to do some dxm with them in an upcoming weekend. I've done the stuff before and don't need any anti-drug talks(I know what I'm getting myself into). However, I've heard that I need to be aware of something called serotonin syndrome. Could someone please give me an in-depth description of this? I'm currently on Zoloft 100mg for social anxiety, as well as a few vitamins/herbs including St. Johns Wort. What are the chances that I may develop this problem? Also, what are the consequences of developing serotonin syndrome? Is it a one time overdose issue or like a long-time physical/mental issue? Should I get off the ssris/herbs before I take DXM? Please answer these questions and anything else important that I should know. I want facts, not lies; however, please don't hold back if I'm missing something important. Thanks.