I need forehead, filler. I'm scared of going blind. If this were to happen it would be immediate?

I am certainly empathic to the obvious extreme concern and fear evident in your question.  So, first please calm down. While there is a concern that some of the injected material can enter a blood vessel directly or compress a blood vessel to sufficiently lead to problems, this is happily quite rare. Millions of people worldwide undergo these types of aesthetic filler injections every year without problems and with extremely gratifying results. This in itself should be reassuring. Experienced injectors are expert in the anatomy of the face and in the techniques of injection that minimize complications and maximize aesthetically quite gratifying results. The metric I personally use for any medical treatment or technique is "How would I proceed if this were myself or a close family member?"  And, the answer is that I routinely treat myself (yes, you read that correctly; I treat myself in front a mirror in my treatment room) and my family with neuromodulators and injectable fillers in my forehead (and face, neck, hands, etc.) and have been doing so for decades, since the introduction of the original Zyderm collagen fillers in the 1980s.Without more specific information, I can only comment in general terms, but if dynamic wrinkles of the forehead (i.e. worry lines) are the problem, then neuromodulators, such as Botox are probably needed, and these have no relation to the problem you are concerned with. If static wrinkles, i.e. etched-in lines from years of muscular movement are the problem, then a filler, such as Belotero Balance or Restylane Silk may be used. Deep volumizing fillers, like Radiesse and Voluma, are to be avoided in the frown line area between the eyebrows, but superficially injected fillers, such as Belotero Balance or Restylane Silk, especially when injected slowly, very superifically, and in tiny amounts, can be quite effective and possess very little risk of eliciting problems. Fat, being very viscous and requiring deeper injection, would be a no-no in my opinion. Of course, it should go without saying that you should only consult and be treated by a physician injector with experience and expertise. And be sure to ask to see his/her before and after photos.

I frequently do fat injections for the forehead. and I love the results. it works and it can take years off of your face. best is do it in conjunction with Botox. I have been doing these injections for a while now and I have never had problems (yet). the key is the experience of the surgeon and their injection technique. I have seen and treated filler complications on the forehead. I agree with your concerns they are valid. but have faith in the fact that such types of reactions do not occur in experienced hands. 

Lines in the forehead tend to be "dynamic" wrinkles/lines (ie, due to the action of the local musculature) and therefore are usually best addressed using botulinum toxin injections (Botox, Dysport, Xeomin). We occasionally will also use some filler in this area but it is less common.You are correct that blindness is a potential but very rare complication of filler injection. It is very important to see an experienced injector who is comfortable using the various filler products in different facial regions.