Tirion had become governor of Hearthglen (this is long before the Scarlet Crusade), living a pretty swank life with his wife and son in Lordaeron. Doing, you know, governor things. Probably Paladin stuff too, I guess.
That changed juuust a little bit when Tirion accidentally discovered Eitrigg, an Orcish hermit that had been hiding away in a broken, abandoned building. Tirion's first reaction was, obviously, to kill the crap out of him. You can't really blame him, the Orcs just got done busting through the Dark Portal like the Kool-Aid Man through your kitchen wall, proceeding to scream "Oh yeah! Oooh yyeeaah!" while they stomped across Stormwind and up into Lordaeron. I would hit Eitrigg with a hammer if I bumped into him after that, too.
Unfortunately for Tirion, the giant pitcher-esque march of the Orcish Horde left this particular building really, really unstable, and the fight between Tirion and Eitrigg brought part of it crashing down on top of the Paladin. Something like that probably would have killed him, but he actually woke up just fine a few days later, tucked into his very own bed. Tirion's right hand man, Barthilas (yeah, that Barthilas), informed him that they'd found him beaten and bloody. Not under a pile of rubble, though. He'd been strapped to his horse and sent back toward Hearthglen, so he'd make it back to civilization without revealing the Orc's location.
Tirion, curious as to why he was saved by the Orc, went back to find him again. Lordaeron and the Silver Hand expected him to "deal with" the Orc. He succeeded in finding the orc, of course. Eitrigg explained to him what the Orcs used to stand for: Shamanism, honor, all of that jazz. This particular Orc had left the Horde due to its corruption. Tirion understood, and promised he wouldn't tell the world where to find him. He lied through his teeth to the Hand and others, telling them that the Orc had indeed been dealt with.
Good ol' Barthilas, ever ambitious, didn't believe him and roped Saidan Dathrohan into tracking down the Orc. They managed to find Eitrigg, and Tirion Fordring was right there fighting for the Orc's freedom, going up against Dathrohan's soldiers. The Orc was ultimately captured, and Tirion as well. Barthilas made sure that Tirion's actions were labelled as treason. He would stand trial at Stratholme for it.
Tirion, against his family's wishes, told the jury exactly what went down, sticking to his guns. Hoping to set an example for his son Taelen and Paladins everywhere, he told it like it was. The jury was not exactly lacking in powerful figures, made up of Archmage Antonidas, Arthas Menethil, Alonsus Faol (founder of the Silver Hand), and Daelin Proudmoore. Ultimately, they decided Tirion had indeed commited treason by assaulting Alliance soldiers, likely more out of social stigma than Tirion actually being in the wrong. He was to be stripped of the Light and exiled from Lordaeron.
Tirion was sent from the jury, and Eitrigg was up next for judgement. Tirion left Stratholme, then rode back later to save the Orc from execution, knocking heads left and right. Tirion was brought down though, not actually being of much use... until a warband of Orcs showed up, which served as a distraction. Thrall, the new Warchief of the Horde, helped Eitrigg and Tirion flee Stratholme.
Eitrigg escaped with like 1 HP and needed mad heals to survive, so Tirion called on the LIght to heal him. The Silver Hand is apparently incapable of actually stripping someone of the Light, and Eitrigg was healed. Thrall invited Eitrigg into the New Horde which was attempting to return to its Shamanistic roots. He accepted, and Tirion continued his life of exile.
From a distance, he watched his family. His wife faked Tirion's death to prevent Taelen from being shamed by his father's exile. Tirion was more or less absent throughout the Third War as far as the big picture goes, mostly keeping to himself. Afterwards, he was horrified to learn that Taelen had been recruited into the Scarlet Crusade. He helped his son through his time in the Crusade from a distance, recruiting adventurers into doing his dirty work. Helping Taelen in battles against the Scourge, giving Taelen safe passage through various places from behind the scenes, things like that.
As of the beginning of Wrath of the Lich King, it will have been about two years ago since Tirion Fordring asked adventurers to help him free his son of the Crusade's grasp. This task ultimately ended in Taelen's death as he tried to escape Hearthglen, which inspired Tirion to end his period of exile. The jury that exiled him is pretty much completely dead. What are they gonna do about it? I guess there's Arthas, but Tirion wants to kick his ass anyway. When Tirion made this decision, he declared that the Silver Hand, which had been disbanded during the Third War, would be reborn, bigger and badder than ever.
Wrath of the Lich King spoilers beyond this line. If you want to avoid spoilers, you can stop here!
Tirion went missing through The Burning Crusade, doing who knows what. I like to think he was off on a two year long training montage, less like Rocky and more like Dragonball Z, because holy cow he was gone a long time. It paid off though, because when we first see him in Wrath of the Lich King he's rocking out in full Tier 6. I'm almost positive they designed Paladin Tier 6 specifically for this purpose, because could you really see the guy in some other Paladin tier gear? Sure, there's Judgement, but as awesome as it is it's kind of old hat by now. I even like Crystalforge and I would burst into gigglefits if I saw Tirion in that.
I won't detail Tirion's entire first appearance in Wrath, because it's way too awesome. Giving away the end result won't ruin the entire scene, so I don't feel too bad doing that. The scene itself is just way too epic to ruin. This much, I will say: Tirion Fordring with the Ashbringer makes me squeal like a little girl at a Jonas Brothers concert.
Eitrigg has apparently not forgotten what Tirion did for him, because he's up in Northrend helping Tirion's cause, too.