If ${\bf X}\sim \text{MVN}({\bf \mu, \Sigma})$ and ${\bf x} \in \mathbb{R}^n$ then:
$$f(x) = \frac{1}{{(2\pi)}^{n/2}\sqrt{|{\bf \Sigma}|}}\exp[-\frac12 {\bf (x - \mu)^T\Sigma^{-1}(x - \mu)}]$$