Warsmith (not to be confused with Warpsmith) is a rank/title employed by the Iron Warriors, given to the leaders of the legion's various Grand Companies. The Warsmiths each have a number of captains who in turn command a regular Company, placing the Warsmiths in positions akin to that of Chapter Masters. Warsmiths are highly skilled individuals, both in skill of arms and at siegecraft. They must also maintain a certain degree of savvyness amongst their equals and direct subordinates: if a Warsmith is killed by a fellow Warsmith, the victor might absorb his opponent's Grand Company, and a good number of Warsmiths obtained their rank by killing its previous holder.
The Iron Warriors Legion of the late 41st Millennium is organised into a number of large warbands known as Grand Companies, each commanded by a Chaos Lord who bears the title of Warsmith. Originally, each Grand Company would have been similar in size and organisation, totaling approximately a thousand Space Marines, but now they vary in size enormously after ten millennia of battle and losses. It is uncertain how many Grand Companies there are at any given time. At the time of the Horus Heresy, the IVth Legion had an added layer of organisation known as a Grand Battalion, which contained several Grand Companies and was commanded by an officer with the rank of Warsmith. At the time of the Drop Site Massacre on Istvaan V there were at least twelve Grand Battalions in the IVth Legion, although with the widespread deployment of many garrison forces in that era, it is impossible to be sure.
It is interesting to note that the position of Warsmith can be held even when its holder is interred in a Dreadnought, assuming they don't go insane (well, too insane by Chaos Space Marine standards). An example would be Berossus from Graham McNeill's Dead Sky, Black Sun. Another would have been Chengrel, a character from a novella, but he had to settle for Chaos Lord since he never officially made it to Warsmith.
It's also interesting to note that every Warsmith other than Honsou tends to get killed after only one novel (Aside from that one who ascended to Daemonhood).