Obsidian and his consort Strika have fought for Cybertron in countless battles... and won them all. Though he is sophisticated and canny, able to concoct strategies with layers upon layers, his purpose is single-minded. He defends Cybertron.
There is some fine print. Whoever happens to be in charge, whether a benevolent or malevolent force, in his reasoning, is Cybertron. Once power changes hands, so does he. He is loyal only to the planet, and perhaps his consort Strika, not to any moral ideals. As his fellow Vehicon General Thrust has remarked, "If you're loyal to everybody, can you really be loyal to anybody?"... a question that has given Obsidian pause.
Tired of the Maximals using his world as a playground and getting fed up with Thrust's inability to handle them, Megatron selected two Sparks to be his new generals. When Thrust first saw Obsidian and Strika, he remarked "Very nice, boss. Very very nice." However, the feeling wasn't mutual, as Thrust discovered when he started to inform the new guys of their enemies - and was batted away by Obsidian, bouncing off the wall and sliding to a halt as his "feet".
Obsidian was quick to act against the Maximals, ambushing Silverbolt and Nightscream with Strika. Screaming "Obliterate!" repeatedly, he pursued Silverbolt but was tackled by Optimus Primal, sending both crashing into the ground. When Optimus tried to reason with him, Obsidian merely fired at him and transformed, escaping into the city. He rendezvoused with Strika and let the Maximals into a trap, as hundred of drones modeled after the two generals surrounded them. Transforming to robot mode, Obsidian remarked "Although he took his name from a great leader, Optimus Primal never learned the first rule of warfare: NEVER underestimate your opponent." The Maximals were able to escape the trap, but found all of their escape routes blocked by drones, Rattrap surprised at the number of drones and Optimus realizing that it wasn't number but tactics; their new opponents were master strategists. Forced to barricade themselves inside the Counsel Citadel, Primal realized that they had been herded there, at which point Obsidian contacted him and offered the Maximals the chance to surrender themselves. Revealing who he and Strika were, Obsidian informed Optimus that "Megatron IS Cybertron. To defend one is to defend the other. And to betray Megatron is to invite death." Despite their advantage, Obsidian and Strika were unable to capture the Maximals due to Botanica's timely arrival, disrupting the drones' targeting systems and allowing the Maximals to escape.
Obsidian's next major move was against Cheetor, Blackarachnia and Silverbolt. Sent to a shell yard by Megatron and told to wait for the Maximals (as Megatron had sent Optimus a flash about the Sparks and their possible locations, which of course was a trap), Obsidian attacked the trio, outgunning and outflanking them easily. However, a power core thrown by Cheetor was hit by a shot from one of Obsidian's drones even as he was desperately telling them to cease fire, causing a huge explosion that sent him and his drones flying away in a fiery shockwave.
When Megatron began to enact his plan for the Sparks, Obsidian and Strika were stationed to watch over the ground and air around the BFH. While he didn't get to do much personally, Obsidian attempted to engage Savage/Noble in single combat, only to have one of his own drones smacked into him and then Strika thrown into him. Ouch.
After Megatron's seeming destruction, the generals were at a loss. Without a leader they had no purpose, and when Cheetor and Nightscream approached them with an offer to join the Maximals they accepted, much to Thrust's annoyance. A possessed Jet Drone suddenly attacked both the generals and the Maximals, destroying the drones that were present. Obsidian and Strika did not join Cheetor and Nightscream in the fight against the Jet Drone (wusses). Later, Optimus welcomed the two to the group and sent them to patrol the inside of the Gran Mal, where they unsuccessfully tried to stop Megatron's depolarized Spark from entering. After Megatron was re-polarized and bonded to a Diagnostic Drone, Obsidian and Strika seized him and made their escape, pledging their loyalty to evil once again.
The conflict came to a head when Megatron began an all-out attack on his former base, seeking to reclaim the Sparks. Obsidian fell victim to Rattrap's identity scrambler, destroying several of his own troops before realizing his mistake. He later returned to his main fleet that was attacking the BFH, but they were unable to penetrate the shields. That changed when Megatron found a device Rattrap used to send Botanica back to the surface - a shield harmonic device synchronized to the base's shields. Obsidian and Strika were then able to fire through the shields and destroy the base's drive units, sending it plummeting into the Citadel.
After repairing and placing his troops, Obsidian began a second attack on the Gran Mal with Strika and Thrust. Taking down the shields and defeating several Maximals, the trio were left with only Cheetor in their way. However, the Maximal led them into the engine room, where he brought the anti-gravity generators online and sent Obsidian and Strika into Cybertron's orbit.
On an alternate Cybertron, Obsidian fought alongside the Decepticons Tankor, Razorclaw, Reptillion [sic], and Blackarachnia against Optimus Primal and his Autobots. One of these battles—the most violent of that year—was interrupted by a mysterious ship that sucked those Decepticons up on beams of light and sped away. None of the combatants realized that it was taking its captives to Unicron for use in a massive army.
In a parallel universe, the Predacon Megatron "decimated" Cybertron's population by capturing their sparks, including the vaunted Autobot general Obsidian. (It is unknown whether this literally means he only got one-tenth of the Cybertronians.) When Megatron gave him a new, helicopter-based body, Obsidian hid his thirst for revenge behind a feigned loyalty. He conspired with the brute Tankor, eventually using the latter's raw strength to carry out Megatron's termination. Obsidian, now an "ex-Autobot," supplanted the Predacon's rule with an iron-fisted tyranny of his own. With his enforcer Tankor at his side, he put down uprisings and held the planet in fear... until Unicron decided that such a mindset would serve his own purposes, and he pulled them out of their universe entirely.
Unicron, his ruined form trapped between dimensions, sought to revitalize himself by pitting abducted Transformers against each other within his own body, absorbing their energon radiation and, eventually, sparks. But Obsidian was spared this fate, instead overseeing the deathmatch arena along with Tankor, Razorclaw, Reptilion, and Striker. Silverbolt, one of the intended victims, recognized Obsidian. Obsidian confirmed the familiarity, but noted that in his and Tankor's own timeline, Silverbolt had "died screaming."
For all his boasting and accomplishments, though, Obsidian still did a pretty crappy job of keeping Optimus Primal from freeing Silverbolt and dozens of other captives. Notably, by this time Obsidian's allegiance had fully switched to "Decepticon," though it's unclear if this was before or after his coming under the thrall of Unicron.
The Tripredacus agent Flamewar was held prisoner and interrogated by a group of Autobots and Maximals which included Obsidian. However, it transpired that the questioning was merely a test of suitability by the Predacon insurrectionist Megatron, and the entity appearing to be Obsidian was in fact a disguised Buzz Saw.