Secutarii

The battle in which the destruction of the Legio occurred was a savage affair, with the Legio Congelatio squaring off against a Traitor Guard force of far greater size.  The war engines of the Legio were able to drive off the scout and forward armor elements of the Traitor force without too much trouble, but the pressure applied by the Chaos force was relentless.  The Corpus Secutarii forces protecting the Titans encountered rough terrain as the Titans advanced, and the Titan crews were left with a coice: abandon their escorts, or allow themselves to be slowed.  The Legio chose the latter, and endured a punishing Manticore barrage from the Traitors.  The final problem was that of a Chaos Titan force that accompanied the Traitors, from a Legio that remains unidentified.  The Titan force consisted of four engines, accompanied by a pair of Chaos Shadowswords.  As soon as they came within range, the Congelatio Titans engaged with the super-long range energy weapons that were the hallmark of the Legio, and managed to destroy a Ravager before it could return fire.  However, just as the Chaos force and the Loyalist force entered direct battle with each other, a third party entered the fray.
A wing of eighty-five Dark Eldar aircraft armed with Void Lances appeared at the edge of the engagement.  Raking the assembled Titans with their dark energy beams, the Titans withered under the unimaginable technosorcery of the evil xenos.  Chaos and Imperial Titans alike fell, destroyed or crippled, as the Chaos artillery kept up its savage barrage.  When the Black Winter Warlord Titan of Cognomen force detonated from a lucky reactor kill, the chain reaction that ensued destroyed all of the Titans left in the fight, which had been forced together by the bad terrain.
The stunned Chaos and Imperial forces outside the blast radius fell back.  The loss of their Titan cover meant that the Chaos artillery could no longer operate so far from their caches, and they turned tail and ran.  The Imperium noticed this and pursued, eventually driving the Chaos force off-world.

Contemporarily, Cognomen has begun to expand its Titan Legion and its Corpus Secutarii.  The Secutarii of all Titan Legions, of course, answers to the leadership of the Legion itself, not its attendant priests or Skitarii.  However, the savage beatings the Legio Congelatio has endured over the years have hammered a sense of fatalistic and even remorseful pride into the Secutarii of Cognomen.  The prevailing belief among the Titanshields – as the local Secutarii dub themselves – is that the world is not at fault for any of the troubles that have befallen the Legion.  Rather, the very sector itself, the untamed wilderness of Imperial space beyond, or even the nature of the galaxy are siding against them.  Secutarii of the Corpus Congelatio paint their armor black with green highlights, and are perhaps the most enthusiastic wielders of phosphor and irrad weapons in the Sector.  Though this is understandably a huge liability in the largely defensive wars of the Sector, the Titanshields demand the right to wield the ancient hate of Mars.
Naturally, this is why the Titanshields are so often rebuked by other Imperial Commanders who do not share their passion for uninhabitable wastelands.  The Titanshields therefore, often reluctantly, field Arc Weapons, Hellguns, and Galvanic weapons when they are fighting on Imperial worlds.