Savage Blow is a powerful downward sword strike, similar in appearance to Power Slash. It is a skill used most often by members of Jolly Roger's Undead Army and EITC Grunts, like the Undead Raider (pictured below). It is currently NOT a skill pirates can possess or use, though the aforementioned similar move Power Slash can be used by pirates.
An Undead Raider about to use Savage Blow.