Chaplain Cassius is an attempt by GW to create a 'minor' tarpit unit for the space marines. To some degree they succeed, but ultimately there is no getting around the fact that Cassius, at his core, is just a chaplain. In third and fourth edition of the game, that would have meant he was a dangerous close combat character. In the current edition of the codex, however, a chaplain is really just a bump in the road.
Basic Equipment
Cassius has all of the core gear that denotes a chaplain: power armor (3+), a rosarius (4++), bolt pistol and crozius (power weapon). This means he does have, at least the basics, for making him a decent hand to hand opponent. He isn't a crazy assault character, but at least the power weapon gives him a little bite. In addition to the basic chaplain equipment, he also carries 'Infernus' -- a combi-flamer loaded with hellfire shells. Frankly, I find this a little odd. Like Pedro Kantor and Darnath Lysander, GW has opted to create a character that appears to be geared towards one role, and then is given gear or special weapons to perform a different role. This weapon is no different. With the hellfire shells, Cassius is actually a fairly decent shooter. The ability to wound anything on a 2+ is a fairly handy ability. I suppose the combi-flamer portion of that weapon makes at least a little more sense in that he can better soften up targets in an assault... but only the little gribbly units that would actually be affected by it.
Special Abilities
Also like most chaplains, Cassius carries the nefarious liturgies of battle, granting re-rolls to hit on the charge for him and his unit. Once again, a very assault oriented ability that could be devastating in the hands of a full fledged assault unit, but kind of odd for a 'meh' chaplain. But at least it gives him some kind of support role. Also keep in mind that as a chaplain, he also "grants" the Fearless USR to his unit -- which has its positives and negatives; that means his unit won't be slowed down by pinning or morale tests, but the dubious No Retreat! rule is present in the assault. For a space marine with ATSKNF, Fearless is really not as effective as the ability to run away from a lost assault and then only be subject to No Retreat! if the other guy catches you.
The biggest 'special ability' is his boosted toughness. Because of various upgrades and bionic prosthetics, Cassius is literally as 'tough' as a monstrous creature at T6. This means a couple of things: (1) you can't double tap him out with Insta Kill; (2) S3 weapons are useless; (3) your enemy will fear him much more than he needs to. Let's face it, with otherwise basic Chaplain stats, he's only got two wounds. So, in effect, the ability to dodge Insta Kill is kind of pointless. I mean, if you're hitting him with something that hits that hard, you're probably pointing at least two of them in his direction to begin with. I suppose that means he may last into the second round if he is in a squad and you get to spread the wounds around a bit, but in an assault, he's probably going to die anyway. Being immune to S3 is also kind of nice, but once again, there is a whole lot of S4 and above weapons on the battlefield that will be more than effective at bringing him down. I suppose the one good thing about being T6, unlike a monstrous creature, is that Cassius has the invulnerable 4++ save. That means at least half the time he'll be able to shrug off those nasty wounds that would otherwise lay him low. But once again, the kinds of things your opponent will be throwing at him will probably be more than apt at bringing a few wounds to the table.
Using Cassius Effectively
This is a tricky section to write because it's actually kind of hard to use a chaplain effectively in the current edition of the codex. By taking away the Chaplain's third wound and boosted combat stats, GW intentionally toned down the chaplain so that he is no longer a hero on the battlefield. Never mind that toning down the chaplain's abilities is actually contrary to the copious tomes of fluff that have been around for thirty years or so... The re-roll in an assault can be handy, but you're only going to get that on the charge, and it's hard to justify spending 100 points or so on a basic chaplain who isn't going to do much more for you than a veteran marine for a third (or less) of the points. Especially when you can get a basic captain, who has better stats and access to better equipment, for roughly the same price.
I suppose one of Cassius' saving graces is that he is the cheapest of all of the special characters available. You can still get a basic captain, master, chaplain or librarian for cheaper, but I guess it's still something if you just have to bring a special character to the field. With his improved toughness, he does at least have the capability to stick around a little longer than your average "hero" provided you don't throw him at something that hits back too hard. In fact, putting him in a unit of Sternguard is not a bad idea at all as he will blend in very nicely with his hellfire shells, and can contribute to the combi-weapon goodness when it's time to counter-charge the enemy with your veterans. In fact, because the sergeant also has access to a power weapon (preferably a fist), it may give your opponent pause when deciding how to distribute his attacks.
Another decent idea is to throw him into a unit of better assaulters than himself. Vanguard units would be happy to have the extra firepower, and the Fearless ability may be helpful when trying to cross the battlefied. However, because Cassius doesn't have access to a jump pack, he may not be the best choice for a Vanguard unit unless you plan on dropping them with a pod or sticking them into Rhino.
Wrapping Up
Cassius, at his core, is just a chaplain, and chaplains are not an impressive 5th edition codex choice. There are far better choices for the points, and his abilities are generally overestimated by both wielder and defender alike. He should last a bit longer than your average chaplain, at least, and has at least a little bit of sting with his hellfire rounds. If you've got the points to spare, he might be a fun and characterful option, but otherwise, you're probably better off with something else.
