"Boyz, wez kan rebild 'im. Wez gotz da teknowragey. Wez gotz the gubbins ta build da wurld's first bionik ork. Skeevin Ashtin be dat ork. Betta dan befor. Betta, stronga, fasta..."
- – Da Six Dozen Toof Ork
Orks are dead killy. They are huge, hulking creatures, obsessed only with fighting and killing whatever is within their reach. Whether it be Shootas, Choppas, Rokkit Launchas, or even the dreaded Power Klaw, the average Ork Boy has a lot of options available to him.
However, to some Orks, a life as an ordinary footslogging Boy isn't enough. Some may try and fail to rise in the ranks, while others may dimly realize that they are only cannon fodder for some Marine's Bolter. Although Orks don't generally fear death, they do fear not being able to kill stuff anymore - to an Ork, that is the greatest fear of all.
Long had the Oddboys been discussing this matter, trying to solve this grave problem. Experiments ensued, all without success. In the end, it all seemed futile, until one enterprising Mek had a brilliant idea, inspired by the bluddy Humies no less. He had been fighting some Space Marines like usual, looting destroyed and cast-off equipment along the way. Suddenly, he saw the most beautiful machine he had ever seen. Boxy, heavily armoured and loud as hell, the machine tore through the Ork Boyz at an alarming rate, destroying everything and anyone, bellowing with a metallic voice, "EVEN IN DEATH I STILL SERVE." The machine was a Dreadnought, a walker with a barely-living Space Marine within its shell.
Seeing the Dreadnought had gifted the Mek with inspiration. He went home and began feverishly building a mechanical walker, arming it with rockets, ripping blades, guns and swords, and equipping it with a cockpit just big enough to house a small Ork. Soon he found such a specimen, a rather diminutive Boy on the verge of death. The Mek had a Painboy hardwire some cables into the Boy's brain, whereafter he interred the Ork into the machine. When the Ork awoke he was filled with Rage, and when he realized what had been done to him he rushed out to show the clan his new, mechanical body by showing their internal organs to them.
The proud Mek dubbed his creation a "Deff Dred," meaning a Dreadnought that is killy even by the extreme standards of the Orks.
Since that day many other Orks have found themselves bolted into a Deff Dred- some willingly, some not. Those who are willing typically do so with the intent of securing a place of high status in the Clan by leapfrogging past even the biggest and toughest 'Boss. Unfortunately, they almost always discover that this is foolish, because the biggest drawback of being bolted into a large, mechanical shell is that you have been bolted into a large, mechanical shell. While Deff Dreds are undoubtedly important assets to any warband, the Dred pilots themselves almost never attain a high position within the simple hierarchy of Ork kulture. Once the interred pilot reaches this realization, the best he can do is to brutally take out his anger on whomever the warband happens to be fighting at that moment. This is, admittedly, something that Deff Dreds do quite well.
On the Tabletop
Unsurprisingly, Deff Dreds are roughly comparable to standard Space Marine Dreadnoughts: in 8th edition they are T7 with 8 wounds and a 3+ save. Their combat ability does not decrease when they take wound damage and by default they come with two Dread Klaws and two Big Shootas. However, they can replace any of these weapons with a Rokkit Launcha, Kustom Mega-Blasta, Skorcha, or Dread Saw on a one-for-one basis. It is therefore possible to construct an all-melee or even an all-gun Dred, but just keep in mind that by the strict wording of these options it is not possible to use either 4 Klaws or 4 Big Shootas (although you could, for example, run a Dred with 2 Klaws and 2 Saws, which is still more than enough to scrag almost anything short of a super-heavy or a Knight).
Dreds do have some significant drawbacks. They are rather slow with a speed of only 6", and they are also large, conspicuous, and meaty targets. For these reasons, in 8th edition there has been some debate about just how effective they are. Supporters of the Deff Dred argue that it is reasonably costed at around 100 points for most loadouts, and you shouldn't ever be taking just one anyway- take two or three at least. For maximum hilarity, take a mob of 3 Dreds optimized for melee and then use the Tellyporta stratagem to plonk them down to within charge distance of the most important and/or vulnerable part of your opponent's army.