Chaplain Cassius is consider one of the best chaplain you can field for the Space Marine codex. For the price of 125 pts you actually get alot of stuff. A regular chaplain without upgrades is around 100 pts.
He has all the features of a regular chaplain along with this extra like he has a combi flamer that fire hellfire rounds, He is toughness 6 and have feel no pain.
So what makes him unique is that he carry a combi weapon and mantain his bolt pistol. Regular chaplain would had to surrender that pistol to gain the combi weapon. So in an assault he can still have +1 Attack for carrying 2 close combat weapons in the field. Another thing is since he has toughness 6, feel no pain and Rosarius. Against weapon like missile launcher a regular chaplain would die on the spot if he fails his 4+ invul regardless of how many wounds he left. Cassius can do his 4+ invul then 4+ Feel no pain. Which this grants him a 75% chance of survival than 50% from a normal chappy. Even his chance of surviving a normal save is 83.333% that almost like a Terminator armor save. So potentially you can actually lock enemies with no power weapons in a close combat, since your enemy have to go though toughness 6, 3+ armor save and 4+ feel no paint while u can slowly slice them with your crozium.
Even the combi flamer he carries is useful, You can use it to soften your enemy for your assault team or if there several squad on the table.. Cassius can split up and burn any weak enemies that hiding in cover. Sparing your assault team unnecessary risk or hassle to take care of them. The hellfire round he carry comes in handy dealing with Daemon prince or Monster Nids. Just stay back and fire away.. force them to make armor save.
Generally my recommendation is to attach Cassius to a squad of assault terminators. Regardless of size preferable no more than 7... since u may need a Land Raider unless u have some strategy that include walking. Get some a good mix of Lightning Claw terminator and Thunder Hammer with Storm shields. Now Lightning Claw terminators enjoys reroll to hit and wounds.. forcing your enemy in a world of hurt. Ideally thunder hammer is more for to allocate ap2 shots on them, Instant Deathing IC or multiple wounds squad or anti vehicle ( Yes Liturgies of battle allow you to reroll to hit rolls on a vehicle.. So even if your enemy move cruising speed.. u can get to reroll that 6+ to hit). If they are MC around, keep cassius away and let him fire his hellfire round while your terminators finish him off.