Jeff Smith stands at a podium in 2015: Heidi Gutman/CNBC/NBCU Photo BankStarboard Value has taken a $1 billion stake in drugmaker Pfizer.The hedge fund, headed by Jeff Smith, has become one of the most feared activist investors.Starboard has doubled its assets over the past decade targeting companies across industries.Pharmaceuticals giant Pfizer got a wake up call Sunday evening when one of Wall Street's most feared activist investors revealed a stake in the company.Starboard Value informed the drugmaker it had taken a position in the firm valued at $1 billion.An activist's aims, whether that's pushing for breakup, M&A, or cost reductions, vary depending on the target.