You know what's cool about Roman legions? They seem to have a collective personality. Generally I have little interest for warfare in history, I'm more often captivated by personalities and characters, but legions are just as good. So, let me share the story of Legio I Adiutrix, and you judge for yourselves.
We're in the year of 68 AD, and Nero's reign of terror is drawing to an end, only to be followed by other people's reigns of terror. Aware that riots started all over the place, confused by the whole situation and incapable of any decisive reactions, Nero attempted to organize some sort of defense at least in the city of Rome, and gathered the sailors from the imperial fleet, which was stationed in Misenum at the time, trying to organize them into a legion.
He gave up his plans to put up a struggle, as we know, and committed suicide before the sailors had a chance to become legionaries. Nero's short-lived successor, Galba, marched into Rome, coming from Spain, in 69 AD, after a trip that had already been stained with blood at almost every stop. He met the sailors, now jobless and without a clear statute, at the Milvian bridge (the same that would be made famous by emperor Constantine).
The sailors asked for the legion to be properly organized, as Nero promised. Galba, a guy without any sense of humor and with a passion for senseless authority, ordered his own troops to charge the sailors. It's also possible that Galba thought the disorganized group had the intention to defend Rome against him in a futile demonstration of loyalty to a different pretender to the imperial throne – confusion was at its peak and emperors were proclaimed over night by the dozen.
The result was a bloodbath, with thousands of deaths.
After having his imperial status confirmed in the Senate, Galba turned his attention to the survivors, and decided to organize the legion after all - Legio I Adiutrix. If you think the hapless sailors could now take a moment to rest, you're wrong: Galba organized them into a legion only to enforce the most famous punishment used in the ancient Roman army: the decimation.
But he wasn't satisfied by looking at the new – and already decimated – Legio I Adiutrix. Now, if anybody doesn't know this, here's a bit of explanation: in order to decide who died when decimation was ordered, soldiers drew straws. This, of course, means that the guilty could survive and the innocent could die. Galba didn't agree with Fortuna on this one, so, after the decimation, he ordered the execution of those considered to be the heads of the rebellion from the Milvian bridge.
You'd say that a legion that had its numbers halved before facing its first real battle wouldn't have a long life, but I Adiutrix proved otherwise. In the same year of 69 they fought bravely, but unsuccessfully, in Cremona, defending Otho against Vitellius, which makes sense: Otho overthrew Galba, a man that surely was not popular with Legio I Adiutrix. Fortunately for them, Vitellius was not vengeful (or realized he was running out of soldiers), so he took no measures against them, just deployed them to Spain, to cull a Batavian rebellion.
Seems the soldiers of I Adiutrix were doing a lot of traveling, because in 83 AD we already find them somewhere near the Rhine, fighting the Germanic tribes under the command of Domitian. The legion's standard was a capricorn, and the legionaries' helmets displayed a dolphin, probably as a reminder of their initial status as sailors. (By the way, becoming a legionary from a sailor was a huge career improvement in those days, kind of shifting from front office to back office jobs in today's banks. As a land soldier, one actually had a chance to live long enough to retire.)
Now, fast forward to 96, again a confusing year – something which seems to suit Legio I Adiutrix. Domitian was assassinated, the aging Nerva took over the empire, and the question of the day was who was going to succeed him. I Adiutrix was now a force to be reckoned, so Nerva bowed to their demands and adopted Trajan as his successor. Later on, Trajan acknowledged their support and gave the legion the cognomen “Pia Fidelis” (the loyal and faithful). See? That's what I'm saying about Roman legions. They even had cognomens. How cool is that?
Trajan took his now most loyal and faithful legion with him to fight in Dacia, where they'd be stationed from now on most of the time, on and off. In 193, we find them among the supporters of Septimius Severus for emperor. (In fact, after the first glitch with Otho, I Adiutrix seems to have pretty good taste in emperors.) The grateful Severus mentions the legion on a coin.
No idea what happened next. At some point, I Adiutrix was named “Pia Fidelis bis” (two times loyal and faithful) and “Constans” (the reliable), so surely something happened, but I couldn't find any info. The last time the legion is mentioned is in 444, somewhere in Pannonia.