The Ragnarok heavy tank was first made during the Krieg civil war, as an alternative to the Leman Russ. This giant metal box was first developed by the regiments of the Death Korps of Krieg during the 500-year-long clusterfuck that consumed Krieg out of necessity. The Ragnarok was the largest and most impressive (For WW1 trench enthusiasts) of the vehicles designed during that conflict and was intended to replace the many Leman Russ tanks that were lost by the Loyalist Krieg forces during their battle against the Traitors.
A knock-off of the Soviet KV-2 with the usual 40K additions of an even bigger gun and no space for ammunition or an engine (a little unnecessary, as the KV-2 was already pretty ridiculous itself, more on that below), Firaeveus Carron would have a fit if he ever saw one. Used by those crazies the Imperium calls the Death Korps of Krieg, siege platoons or any regiment too poor/under-equipped to get their hands on a Leman Russ, this thing is slow, heavy, armoured like hell and capable of dishing out the same punishment as a Leman Russ. Performing surprisingly well for a Kitbash, the techpriests on Lucius decided to tidy it up, dub it the "Mobile Bunker Mk 1" and then start churning them out. This is actually good when used as an infantry tank, like the pre-Heresy Malcador was a good cavalry tank. Look up on some other wiki what those types of tanks are, if you're curious.
In a sense, the Ragnarok was the Siegfried's older brother as they are both kitbashed-turned military vehicles that was given the thumbs up by the AdMech to continue producing so long as they garnered results. Whilst one would argue that the Ragnarok suffers a horrendous high profile even by the standards of the Imperium tanks, it is also cheap and easy to produce off the shelf. Still the Ragnarok needs overwhelming numbers to compensate for the piss poor boxy design and the overly tall profile, if any form of anti-tank weaponry is found than it G-fucking-G cause that Ragnarok ain't coming out in one piece. As the old saying goes, you get what you paid for and the Ragnarok is no exception.
You know your quality-design suck balls if Games Workshop and the WH40K Wikia themselves admit it is one ugly hunk of metal. On the other hand, WH40K Wikia also applauded it for its "solid design" despite the previous statement of ugliness and design incompetence. More proof of WH40K Wikia's Fail. But even calling it 'design' is already a bit of a stretch, since as mentioned above, the Ragnarok is basically a copy-pasted KV-2 Heavy Artillery tank. Without going into too much details, the derpy looking boxy monstrosity saw the day because the Russians needed a self-propelled howitzer like *right the fuck now* during the Winter War against Finland. So they took some of their freshly minted heavy tanks which they believed would be sturdy enough, jammed a 152mm howitzer they had lying around into a turret on top of it, and off it went to the front lines... where the design's multiple flaws became all to readily apparent. Too big, too slow, the gun was so heavy it could jam the turret if it was rotated without due precautions or flip the tank on its side if fired at the wrong angle... You get the idea: it was a rush job (unsurprising since the war lasted but a hundred days in all) and like most rush jobs performed quite poorly. Still, unlike some other idiots, the Russians realized it was no good just as quickly as they designed it and went for a (lighter, more maneuvrable and cheaper) turretless design that did the job of blasting bunkers and fortifications (and the occasional German tank) apart and opening the door for the infantry just as well.
For those that want a Ragnarok but don't want to kitbash it, Maxmini actually has a full kit for one here called the Gothic KV2 Tank. Makes you wonder why Forge World, with the boner they had for the Death Korps, never made one themselves.
Has recently been added to World of Tanks as a premium KV-2. Go figure.