Well, the immortality part is due to a mini-infinity circuit built into the armour, which fuses the souls of everyone who's worn the armour, albeit with the personality of the first warrior being dominant. That was the fluff back when I last read about it at least, I think.
I must admit I haven't looked into this series, and don't know too much about what you're talking about at the moment, but Gav Thorpe has written some great lore in the past, iirc, so hopefully he's got something figured out.