Dreadnought
Dreadnoughts are the siege walkers of the Space Marines. They're piloted by a Space Marine who almost died, but survived only with the Emperor's grace and loads of plot-armor favoritism. If the apothecaries managed to save the life of a marine from wounds that would out-right kill a Space Marine (Which is saying a lot since they can take as much punishment as a heavy battle tank), but can't be restored to battle-capable condition anymore, the marine has the option of being interred into a Dreadnought and pilot it from the confines of his armored sarcophagus.
Dreadnoughts are incredibly tough and their pilots even tougher, they're totally fearless and won't mind that they're gonna die a painful death under the hail of anti-armor fire and the only real way to totally kill a Dread is by blowing it all up or destroying the Sarcophagus.
HOW TO MAKE A DREADNOUGHT
1: take 1 badass Space Marine
2: shoot him in the kneecaps and a couple of his lungs
3: leave the dying body on the doorstep of some Adeptus Mechanicus guy
4: ???
5: PROFIT!!! DREADNOUGHT!!!