Can fluid (+ fluid build up) in the middle ear be fatal or cause serious complications?

It depends: Fluid can collect in the middle year, most often due to infection or trauma. If it is infection it will cause you to have severe pain inside your ear. If there is no pain then it is not likely to be infection(infection can be serious as it can lead to meningitis which can be fatal). But this is only a consideration if you have fever and severe ear pain.

Fluid: Fluid in the middle ear, if associated with an infection, can cause hearing problems and occasionally more serious complications due to extension of the infection. I feel that an Otolaryngologist is best qualified to evaluate,advise and treat any patient with fluid accumulation in the middle ear.