Heavy Intercessors of the Third. Despite their impressive armaments, they serve more as a positional unit, blocking the enemy's advance and acting as an axis around which the assault units can manoeuvre.
One member of the Inquisition was able to confirm what she suspected - the "Third" is not a company number (they refer to their company as the "10th and Final"), but a Legion number. Furthermore she discovered that their Captain has some very old things indeed - ancient armour, a venerable blade, and an old name - Saul Tarvitz.
It remains to be seen how her colleague, Master Inquisitor Tara, will explain that.
Edit: this will also be the last photo in which Tarvitz sports his "III" icon, which has had to be reglued multiple times already and has no chance of reaching a game day intact.