Doomglaive Dreadnought
Maaannnnn....if you thought the Deathwatch Dreadnought was fucking rare, wait till you see the Grey Knights' Dreadnoughts (Or Doomglaive Dreadnoughts in case you want it to make it sound even more 'special'). Just mix in the already rare and difficult to produce Grey Knights and then increase the rarity even more with the already uncommon process of interring a Space Marine inside a Dreadnought. The result is one of the rarest and most OP Dreadnought in the game that is not a golden lube of merciless penetration.
Strangely, Grey Knights don't really view being interred in a giant METHUL BAWKS that honorably, so they aren't too hot with the idea. And fair enough, fighting demons for centuries is probably enough service to be at least be allowed to die and rest. This could probably explain the extreme rarity of these things.
Doomglaive Dreadnoughts can be seen as the more sensible version of the ridiculous Blood Angels Librarian Dreadnought which is hilarious when put in the context of Grey Knight absurdity. They also gain browny points for looking more sensible then that fucking baby carrier in their armory.
Overview
Though fearsome opponents, the greatest gift a Doomglaive Dreadnought can provide is support to their brethren in the form of psychic defenses and as mentioned before, is much more reasonable and understandable then the flying fuckernaughts of the Blood Angels (Thanks Ward). This is because their sarcophagi contain psyber-circuitry which can link their own Aegis field to that of their Battle-Brothers, thus creating a series of wards that are much stronger than the sum of their parts.
It is ultimately considered not the weaponry nor the armour of the Dreadnought that makes them such terrifying weapons, but the warriors within who long ago were wounded to the point that they now dwell within a cyber-organic sarcophagus. As only the mightiest are ever given this honor, fighting them is no simple feat, as their enemies face not a mere machine or a great hero of the Chapter but a warrior whose mechanised form is as unflagging as their will.
It is customary for personal consent to be given before the procedure can begin to place a mortally wounded Grey Knight Space Marine into a Dreadnought. This is because many Battle-Brothers prefer to rest in peace as heroes of the Chapter in the tombs below the Fortress-Monastery on Titan. There, the name of the Grey Knight is engraved in their crypt alongside other great warriors which is a fitting end for their lifetime of service to the Imperium.
Grey Knights Dreadnoughts as such, possess a number of ridiculous FUCK OFF defenses and weaponry. First off, all of them possess Aegis Armour, allowing them to fend off attacks by Psykers and Daemons. Their left arms are usually armed with one Dreadnought Close Combat Weapon or Nemesis Doomfist and a built-in Storm Bolter, which can be upgraded to either a Heavy flamer or an Incinerator. Their right arms are usually fitted with either a Twin-linked Lascannon, a Heavy Bolter, an Autocannon, an Assault Cannon, a Multi-melta or a Plasma cannon. They are also commonly upgraded with Blessed, Extra Armour, Hunter-killer Missiles, Psycannon Bolts, Sacred Hull, Searchlight or Smoke Launchers. For added Rape, they can also be outfitted with a Nemesis Doomglaive and Psycannon just in case you really want to cream those Daemons.