"He knows a lot about killing aliens, which is no bad thing."
Ortan Cassius is an absolute badass.
He is also, coincidentally, an Ultramarine. Luckily, Cassius is so badass that he is immune to Matt Ward's vile influences. However, he cries manly tears that the rest of his Chapter has devolved to such stupidity at the hands of Ward.
In all seriousness, though, Cassius is the Master of Sanctity and head Chaplain of the Ultramarines Chapter. He also happens to be their oldest active member. He made his name during the First Tyrannic War, in which he attempted to save the northern polar fortress and led efforts to reclaim it after the loss of the First Company. This did not go well, and Cassius was (nearly) crushed to death by a rampaging Carnifex. However, he was such a badass that he somehow clung to life while the Apothecaries rebuilt him, Six Million Dollar Man/Deus Ex: Human Revolution style (I guess there weren't any extra dreadnoughts to put him into at the time). His replacement parts make up most of his body, so now he's pretty much immune to bleeding to death and shock. When he woke up, he had a new goal in life: exterminate the Tyranids, which he had come to believe were the Ultramarines' punishment for being such zeros. To accomplish his goal, he gathered the Tyrannic War Veterans, the only other badass thing the Ultramarines have ever produced.
According to Deathwatch: Overkill, Cassius was such a hardcore nid-killer that he was inducted into the Deathwatch. His burning hatred for xenos was already so great that he barely broke a sweat during the inductee's hypno-indoctrination when other Deathwatch Marines usually have to be restrained from destroying the vid-gear. To be fair, the vids probably don't even compare to having lived through one of the worst xenos invasions of the century, knee-deep in the corpses of your brothers on your own homeworld. Cassius leads his own kill-team fighting a genestealer cult on a distant mining colony.
Why is Cassius So Badass?
You may have noted a disturbing trend in Ultramarines special characters. Marneus Calgar is the BEST CHAPTER MASTER EVAR, Varro Tigurius is the BEST LIBRARIAN EVAR, Torias Telion is the BEST SCOUT SERGEANT EVAR, Antaro Chronus is the BEST TANK PILOT EVAR, and Cato Sicarius is the BEST SECOND COMPANY CAPTAIN EVAR (okay, maybe not so much on that one, but he certainly seems to think so). In other words, they're all a bunch of fucking Mary Sues. They have essentially no character traits besides being THE BEST EVAR. There's nothing setting them apart from other Space Marines besides the fact they're just better. They're so absolutely perfect that even Fulgrim himself weeps bitter tears that he can never be as perfect as the Ultramarines special characters.
Cassius, on the other hand, is not the BEST CHAPLAIN EVAR; he's not even necessarily a good Chaplain. But he has an interesting backstory that ties into the Ultramarines' fluff as a whole, he epitomizes the dogged determination that characterized the Ultramarines pre-Ward, he doesn't have plot armour, and he makes both his faction and the faction he fights look good. For instance, he doesn't impale a Carnifex on its own scything talons; he gets torn to shit by it and just barely survives, spurring him to dedicate the rest of his life to destroying every last one of the bugs. This makes Tyranids look like a credible threat for the Ultramarines, unlike the poor Daemons, which just look impotent next to the Grey Knights.
In other words, he's everything Matt Ward's characters usually aren't. Whether this is because his background was already set in stone by the time Ward got his hands on it, or it's a fluke of nature, the world may never know... but at least the world can breathe easy, knowing there is a least one Ultramarine that isn't a complete and utter prissy bitch.
He also patronizes Calgar (solely referring to him as "young Calgar"), which is worth the price of admission alone. Furthermore, whenever the Ultramarines fight the Tyranids, Cassius recruits MORE Ultramarines to his Tyrannic War Veterans group (although by now they're not all from the Tyrannic War), which is slowly but surely growing in size and influence. Old man Calgar has thought at various times about putting his Power Armoured boot down and stopping it, since it strays away from dad's Codex rules, but he reckons that sooner or later, another war on par with the Tyrannic War will come, at which point Cassius' faction will be invaluable.
On the Tabletop
Ortan Cassius is a cheap, tougher-than-average chaplain with a couple of fun bits. He's also apparently part Tech Marine. In addition to the ultramarine's usual ATSKNF and Chapter Tactics, he has Independent Character, Prefered Enemy (tyranids), FNP, and Zealot, and he gets toughness 6 by being about 80% machine, (he got ripped up by a carnifex). His warlord trait, Angel of Death, gives him Fear and makes the enemy roll that fear test on 3d6. He has a typical chaplain's loadout of a Crozius Arcanum, a bolt pistol, frag and krak grenades, a rosarius, and a "master crafted relic" combi-flamer he supposedly built himself. Nevermind why a chaplain is building his own gear, the combi-flamer "Infernus" is a typical flamer strapped to the bottom of a master crafted bolter with hellfire rounds (2+ poisoned).
On the tabletop you're spending 40 points above a vanilla chaplain. What are you getting for this? +2 toughness, FNP, and a master crafted combi-flamer. This is a steal. Just for the extra toughness alone, he's worth that. Even with only 2 wounds the high toughness and FNP will keep him alive though more than terminator armor would, with none of the drawbacks associated with the bulkier armor. If you just want to throw an extra officer into your army or you want a cheap character for a low point game, he might be a good pick.
However, if you feel slightly more inclined to bring outside assistance, you can instead bring a younger, cheaper version of Cassius brought in from Deathwatch: Overkill, which represented a time where he served in the Deathwatch and killed a cult of Genestealers. While more in line with a standard chaplain, he gets more goodies for a rather minimal tradeoff. He gets no bolter, but he can still use Sternguard Ammo. However, until there are more rules for a Deathwatch army coming out, young Cassius is forced to bring in his tactical squad at the LEAST, which is a mishmash of forces.