Synthos is the ultimate creation of the people of Sandokan, the culmination of generations of warfare waged upon themselves. Synthos is a living weapon created with the Dark Heart of Sandokan. His telekinetic power can turn anything around him into a weapon, making him very difficult to destroy. After 1000 stellar-cycles of civil war, Synthos represents the hope of all Sandokan— A Weapon to End War.
And end it he did.
Synthos arms himself with deadly plasma emitters, though he is capable of controlling literally all of the Sandokan's vast military infrastructure, including weapons capable of reaching out into local space. Though Synthos possesses vast destructive power, his sensory systems are susceptible to deception by cloaking devices, which can cause him to adopt a more cautious approach.
It is implied that Synthos could survive without the Dark Heart of Sandokan, though this would vastly reduce his abilities.
Shortly after coming online, Synthos went mad and slaughtered the entire population of his home planet. A weapons system with no 'off' switch, he prowls Sandokan's blasted lifeless landscape in search of new prey, even looking out into space, searching for a means off the planet so that he can fulfill his purpose— destruction— on a more cosmic scale.
When the Decepticon bounty-hunter Astrotrain came to capture the Autobots' cargo, Synthos struck a bargain for Force Chip technology, promising to steer clear of Decepticon territory when he escaped Sandokan.
However, once Astrotrain acquired the Force Chip, he attempted to betray Synthos and steal the Dark Heart for himself. Synthos was saved by the timely intervention of the Autobots, who feared the result of combining the Dark Heart with a Force Chip. Astrotrain was (in turn) rescued by Cannonball's pirate crew, and Synthos was rescued by the Autobots. (Sort of, one could argue the Autobots were actually rescuing the Decepticons from Synthos.)
Cannonball later succeded in acquiring the Dark Heart. Synthos's final fate is unknown.