Torias Telion is a Scout Sergeant of the Ultramarines Chapter and a grizzled old fuck. Having been introduced in the 4th edition Space Marine Codex, he is, of course, the BESTEST SCOUT SERGEANT EVAR. In fact, he's SO GOOD, he's loaned to other Chapters to train their Scouts (although the new Codex limits this to Ultramarines successors). Telion is the least annoying of the various Ultramarines special characters (besides Ortan Cassius), probably because he has a badass beard. His characterization in novels by Nick Kyme and Graham McNeill has also been pretty consistent as not giving a damn what the Codex Astartes says about organization and tactics, which leads one to wonder how exactly he still has his position of authority. Even Ortan Cassius almost got the boot for his codex deviance, and that guy is a personal friend and mentor to the chapter master. Ultramarines love their codex. Essentially the I'm too old for this shit guy for the Ultramarines.
He is outclassed in quality of hair and quantity of bitching and moaning by Cyrus. Telion is however less of a bitchy little cunt than Cyrus, and has infinitely better facial hair. Just look at that Hemingway beard.
On the Tabletop
In-game, Telion is a 50 point upgrade for a scout sergeant that comes with a full suite of special rules and a few toys. He can also be your warlord, which is... cool? I guess? This dude has an abundance of special rules, including the smurf's typical ATSKNF and Chapter Tactics, plus he has Independent Character, Move Through Cover, Infiltrate, Scout, Scout Sergeant, Stealth, and Voice of Experience (AKA "Dammit boy, here, let me show you," allowing him to forgo shooting to give another member of his squad his BS of 6), Eye of Vengeance (essentially gives all his shots Precision Shot). If, for some bizarre reason, you want him to be your warlord, his trait is "Storm of Fire," which lets him give any squad within 12" Rending. Neat. He comes to war packing scout armor, a bolt pistol, frag and krak grenades, a camo cloak, and his relic stalker-bolter "Quietus," a heavy 2, rending, pinning rifle.
On the Tabletop Telion does exactly what a scout sergeant should do: he sits back and snipes special units while teaching his carton of noobs. His precision shots rule with his rifle means that he's good for sniping the special guys out of squads (special weapons, manners, icons of chaos, squad leaders, etc) or he can forgo shooting to show one of the pups how to shoot his damn gun (rocket launcher with BS6 anyone?) He has stealth, so stick him in some ruins, have a techmarine buff up that ruin's defenses, and enjoy your 2+ cover saves. As far as weaknesses go, he's an Independent Character with no invuln save and 2 wounds, which could be annoying in kill points games (this will also be a problem if you decide to make him your warlord) and given how common ignores-cover firing is in the modern codecies, that 2+ cover isn't as stellar as it might have been.