An embolism can definitely be fatal. An embolism is a clot of blood, or another matter such as air or fat. It forms and becomes lodged in the bloodstream but then at any moment can unlodge and travel up the body to another more dangerous location, like the lungs. It can lodge in the artery of the lungs or even go to the heart or brain.
It is usually when it lodges in the lungs, heart, or brain that an embolism can kill you. It can cause you not to be able to breathe or can casue blood to stop flowing to your brain, resulting in a stroke or even death. An embolism can also cause a heart attack which can be fatal.
Blood thinners can help reduce an embolism from forming in the first place but if one forms, if an anticoagulate isn't effective, surgery may be required. It is important to do whatever is necessary in order to do away with the embolism before it does cause death.