The Slug is an exceedingly large Necromorph and the boss of the eighth level of the game Dead Space. The Slug was blocking the blast doors on USG Ishimura, which were needed to broadcast a futile message to warn the USM Valor to not open the escape pod they had retrieved because of a Slasher inside.
The Slug had to be fought using one of the Ishimura's anti-asteroid cannons, specifically ADS Cannon 48. During the fight, it would use its five tentacles to hurl objects at the tower, with each tentacle having a weak point appearing as a bulbous yellow mass close to the Slug's body.
Defeating the Slug required the destruction of all five tentacles - while at the same time using the cannons to destroy the incoming debris thrown by the Slug.
Immediately following the Slug's demise, it releases its grip on the Ishimura and begins floating off into space. At which point Kendra Daniels is able to broadcast a warning to the USM Valor, telling them not to open the escape pod they retrieved. The return broadcast shows a Marine of the Valor being slain by a Necromorph, indicating that they lost the battle. The Valor then crashes into the Ishimura, causing considerable damage.
The Slug was originally thought to have arrived on the ship after the initial infection, due to the fact that ADS Cannon 48 would have engaged it. However, Dead Space: Extraction proves this point incorrect; as Nathan McNeill disabled this particular ADS cannon (and possibly the system itself), the cannon would have been unable to fire at anything until Isaac Clarke came to manually reactivate it.