Transformers Zone OVA, manga and story pages
Violen Jygar, a crazy supernatural creature and leader of the Decepticons, ordered his Nine Great Demon Generals to steal the Zodiac so he could possess its great power. According to Dai Atlas, the Zodiac was an all-powerful energy source which supposedly created the whole universe and was the source of all life. It rested in a chamber deep beneath the surface of Planet Zone, where the Autobots were situated.
Violen Jygar sent King Poseidon, Trypticon, Devastator and Predaking to steal the Zone before the Autobots could get their mits on it. The Decepticons nearly succeeded, but the Zodiac managed to fall into the hands of Dai Atlas and Sonic Bomber. Dai Atlas opened the Zodiac, unleashing a surge of its awesome power, which destroyed King Poseidon and Trypticon.
The Zodiac was eventually obtained by Violen Jygar and was the cause of a great battle between the Autobots and Decepticons. By the battle's end, Violen Jygar was destroyed by Big Powered (the combined form of Dai Atlas, Sonic Bomber and Road Fire) and the Autobots regained the Zodiac once again.
The Battlestars story pages
When the Decepticon Dark Nova brought Optimus Prime back from the dead as the sinister False Convoy, the Autobot Sky Garry knew that the only hope for the good guys was to defeat False Convoy and bring the real Optimus Prime back online. Sky Garry managed to rescue Prime's lifeless corpse from the Great Novaroid forces and restored him to life by placing the Zodiac in his chest (cuz it's just that easy). The power of the Zodiac turned Optimus Prime into the powerful Star Convoy.