up for sale now (£170 like the other Cerastus pattern Knights) and it's rules are posted as well. (link takes you straight to the PDF) Is this the mastiest of the FW Knights?
So what does this Knight offer? For a mere 415 points in your army's composition, it brings a Reaper Chain Fist which is much the same as the Reaper Chainsword with the exception of having the Machine Destroyer special rule. This allows it to reroll 1s on the Destroyer Damage Table when attacking anything with an armor value. So with the standard Knight Stat line of 4 Attacks, it's is very likely o destroy whatever you are attacking. It also has a built in Twin-Linked Heavy Bolter.
The Heavy Bolter pales in comparison to the Acheron Pattern Flame Cannon for Infantry clearning. It's S7 AP3, but it uses the Hellstorm template. You can get some serious chaff clearing done with that thing.
So what niche does this Knight fill? It has a very heavy anti-Marine equivalent role. At S7 AP3, you will clear Marines and anything else with armor less than Terminator equivalent off the board in a hurry. Granted the Flame Cannon is Ordinance 1, so it's only one shot, but the Ordinance rule makes it effective against less armored vehicles as well. AV 11 and 12 vehicles don't want to get near this thing. The Chainfist also makes it great for taking down other Knights, Fortifications, and Heavy Vehicles. However, you'll want to keep it out of CC as much as possible as it can be tarpitted as easily as any other Knight, unless you have a Castigator nearby to help out.