Unbelievable as it may seem, there are times when even Astartes battleplate simply does not offer sufficient protection. Most frequently, this will be in circumstances where they are required to prosecute undertakings in theatres regarded by the Codex Tactica Imperialis as Zones Mortalis. Such environs include storming breaches in fortifications, void-craft boarding actions, the clearing of bunker and tunnel complexes and the pacification of space hulks.
In some cases, these actions may be undertaken by Breacher Squads augmented with boarding or breacher shields, but in circumstances where the probability of survival is rated particularly low and protection is therefore of utmost concern, Tactical Dreadnaught Armour is likely to prove the best option.
A product of the Terminator programme which sought to create the ultimate protective armour for the Legiones Astartes, Tactical Dreadnaught Armour harnesses the capabilities of civilian exo-environment suits that are able to withstand such lethal environs as those found within the containment fields of macro-plasma reactors by combining ceramite and plasteel plating with adamantium bracing.
According to Kordhial's Liber Armorum Imperialium, several patterns of Tactical Dreadnaught plate were developed by competing armourers, including Kataphraktii, Tartarus, Indominus and Saturnyne with each variant possessing uniquely defining characteristics and appearance but ultimately offering roughly analogous levels of protection to the wearer.
Such protection came at a price though and an Astartes must undergo extensive training before being certified to fight wearing a Tactical Dreadnaught Armour as the bulk and reduced agility of the suit can wrong foot the inexperienced and render even the most decorated of veterans vulnerable to more numerous or more lightly armoured foes.
But to master Tactical Dreadnaught Armour is to be able to walk unscathed through storms of shot and shell that would prove the death of a lesser armoured warrior.
Illiatariu. C., Rememberancier Documantist Minoris. To Walk Among Demigods. A Treatise Upon The Nature of The Argyntum Stellae., Published 863.M33, Sector Heliopolis, Segmentum Pacificus.