This marine is a HQ choice for the Salamanders legion. He was one of the first generation chaplains of his legion and spread the "Voice of Fire" everywhere he went, becoming one of Vulkan's most trusted at the same time. The interesting aspect here, is that in the background Vulkan nominated him as regent of Nocturne. Hence he survived Isstvan by virtue of not being there.
In the game, Nomus is an enhanced chaplain HQ selection. He gives the inspiring presence warlord trait which is okay but not brilliant, by default. It is usual that it is his command that also wakes slumbering dreadnoughts. Hence as a force multiplier, he allows the selection of an additional HQ choice dreadnought talon as a non-compulsory selection. Hence with this HQ choice, the Salamanders player is probably going to be gunning for a dreadnought heavy force, presumably all armed with heavy flamers or the like. Add in Cassian Dracos and you have the making for an amazing force.
Finally, he doesn't have a crozius, so much as a Vulkan forged hammer to denote his rank. Naturally.
He already has an iron halo, so could potentially also add a dragon scale storm shield for added effect. But this starts bumping the points up. Already over 200 points at baseline, this is an HQ that you would want to build an entire army strategy around if you could. Hence he's not simply an HQ to be selected lightly. He's a characterful selection that must use the rest of his army to best exploit his dreadnought bonus. Otherwise, go for someone else.