The Horus Heresy isn’t all about the Dreadnought’s, but they sure are a big part.
The Word Bearers traitor dreadnoughts were both added as part of the build up for an event army. For the longest time Warhammer 40,000 had only the ‘box’ dreadnought (Castaferrum to give it the proper designation) and that was about it. The heresy series however brought us the ‘Contemptor’, ‘Mortis’, ‘Deredeo’ and ‘Leviathan’ classes. Some of the names may be familiar as they are from Warhammer’s distant “Rogue Trader” past and pre-date even the Castaferrum.
Two of these were a good fit to provide heavily armoured close support and long-ranged anti-armoured firepower for the Word Bearers, those being a Leviathan and Contemptor.
The Leviathan is the heavyweight of the two with incredibly tough armour. I have faced these in zone mortalis games and short of a melta-bomb there’s really not a lot legionary infantry can do against them. In one of its first games it single handedly eliminated an Imperial Fist cataphractii armoured terminator squad with graviton and phosphex. The more exotic weapons prevalent during the great crusade era another thing that makes collecting heresy armies great.
Equally the Contemptor is armed with a conversion beam-cannon, a deadly but complicated weapon which is more effective the further away the target is.
There is one other dreadnought that is unique to the Word Bearers, which I haven’t mentioned yet—the ‘Mhara Gal.’ It is a tainted Contemptor sarcophagus housing the mortal remains of a fallen Gal Vorbak, a dark brethren.
I’m still undecided about adding it to my collection, considering how powerful it is. In many ways, I feel it may be too powerful. For now, however, I believe the legion Contemptor should suffice.