The Ragnarok heavy tank is a frontline armored monster first produced during the Krieg civil war as an alternative to the Leman Russ. This giant metal box was developed out of necessity by the regiments of the Death Korps of Krieg during the 500-year-long clusterfuck that consumed Krieg. 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 persistent Traitors.
The Ragnarok is essentially a knockoff of the Soviet KV-2, with the usual 40K additions of an even bigger gun than what the IRL tank had and no space for ammunition or an engine (a little unnecessary actually, as the KV-2 was already pretty ridiculous itself... more on that below). Firaeveus Carron would have a fit if he ever saw one. The tank is still used by those crazies the Imperium calls the Death Korps of Krieg, siege platoons, or any regiment too poor or too under-equipped to get their hands on a Leman Russ. This thing is slow, heavy, armoured like hell, and capable of dishing out about the same amount of punishment as a Leman Russ. Performing surprisingly well for a kitbash, the Techpriests on the Forge World Lucius decided to tidy it up, dub it the "Mobile Bunker Mk. 1", and then start churning them out. The Ragnarok actually performs pretty well when used as an infantry tank, like how the pre-Heresy Malcador heavy 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 is the Siegfried's older brother, as they are both kitbashes-turned military vehicles that was given the thumbs-up by the AdMech to continue production so long as they garnered results. Whilst one would argue that the Ragnarok suffers from a horrendously-high profile even by the standards of the Imperium tanks, it is also dirt-cheap and easy to produce right off the shelf. Keeping that in mind, the Ragnarok needs overwhelming numbers to compensate for its piss-poor boxy design and overly-tall profile. If any form of anti-tank weaponry happens to be in the hands of the enemy then it's G-fucking-G, 'cause a Ragnarok facing that will not return to base in one piece. As the old saying goes, you get what you paid for, and the Ragnarok is no exception.
Fiction vs Reality
You know your design team sucks balls if Games Workshop and the WH40K Wikia themselves admit it is one ugly hunk of metal. On the other hand, the Wikia also applauded it for its "solid design", despite making that previous statement of the tank's ugliness and incompetent design... More proof of the WH40K Wikia's Fail. But even calling it a "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 many details, basically the derpy, boxy monstrosity saw the light of day because the Soviets 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 (going by the designation "KV-1"... see what they did there?), which they believed would be sturdy enough to act as a vehicle base, threw away their old turrets, jammed 152mm howitzers they had lying around into new turrets on top of them, and off they went to the front lines... where the design's multiple flaws became all-too-readily apparent. The KV-2 was way too big (they stood out like fucking elephants on the battlefields of the Eastern Front), way too slow (think "retarded turtle" slow), and the gun was so heavy that it could (and often did) jam the turret if it was rotated without due precautions, or even flip the tank on its side if fired at the wrong angle... It was definitely a rush job (unsurprising, since the Winter War lasted but a hundred days in all), and like most rush jobs it performed quite poorly. Still, unlike some other idiots, the Soviets realized that it was no good just as quickly as they designed and deployed it, and later in WW2 they went for a (lighter, more maneuverable, and cheaper) turretless self-propelled gun that did the job of blasting bunkers and fortifications (and the occasional German tank) apart very well, and opened the door for the infantry just as well with High-Explosive shells.
For all of its derptastic flaws though, the KV-2 at least has one badass story that could be attributed to its name: A lone KV heavy tank (possibly a KV-1, but a lot of accounts point to a KV-2) found itself behind enemy lines during the Battle of Raseiniai in 1941 after already murderfucking a column of German supply trucks; four 50mm German anti-tank guns and a heavy 88mm gun (basically the heaviest anti-tank weapon that the Germans had at the time) tried to knock it out after it stopped on a road, but the Soviet tank destroyed them all first. The Germans followed up with combat engineers trying to blow up the damn thing with satchel charges, but they only succeeded in (maybe) damaging the tracks a bit. Holy shit. Finally, the morning after the first encounter with the KV, the Germans sent tanks in front of it as a diversion while another 88mm tried to launch shots into the heavy tank from the rear. Out of several shots made by the BFG, only two managed to even penetrate the rear of the KV (keep in mind that the rear of a tank often has the weakest armor). Finally German troops zerg-rushed the tank and knocked it out by tossing grenades into its hatches once its main gun ran out of ammunition, but not before the tank's crew mowed down a whole bunch of them with the mounted machine guns. The fact that the Germans ended up burying the Soviet crew with full military honors really hits home how fucking amazing it was that a single KV managed to hold out against such overwhelming odds for more than an entire day.
This is one of very few instances where a Warhammer entity cannot match up to its real-world counterpart in terms of how goddamn manly it is.
The Tank in Other Mediums
For those IG players who 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 have for the Death Korps, never made one themselves. For a cheaper option, Blood and Skulls Industry makes a big boxy KV-2/Ragnarok turret you can slap onto a Russ or Chimera chassis.
The Ragnarok has recently been added to World of Tanks as a premium KV-2 in Valhalla Ice Warrior regalia. Go figure.