The Osiron Class Dreadnought was a type of Contemptor used by the Thousand Sons. These dreadnoughts were created by Magnus himself, and consisted of a mortally wounded psyker Space Marine laced with a psychometric barrier for his brain. Called the Osiron device, this allowed psykers to be interred in a Dreadnought without going insane, as well as still having full access to their psychic abilities.
As such, these Dreadnoughts were equipped with Force Weapons. Think of them as the Doomglaive Dreadnought of the Great Crusade and Horus Heresy. Essentially speaking, Magnus had psychic dreadnoughts before it was cool. Magnus, being the bro that he was, even gave the schematics of the Osiron device to all of the other Space Marine Legions, though it would appear only the Blood Angels and Grey Knights ever made use of it.
For many solar decades the arts by which the power of an Astartes psyker could be maintained medically within the armored sarcophagus of a Dreadnought eluded the Thousand Sons and any other Space Marine Legion that tried, with the occupant experiencing a lingering death, madness or malign psychic phenomena. The Osiron Dreadnought with its device was aimed at solving this issue.
The Osiron device was difficult to construct and was barely understood by even the most learned psi-arcanists, though this did not stop the Thousand Sons from outfitting it to their Contemptor Pattern Dreadnoughts, with startlingly powerful results.
The Osiron Dreadnought mirrored the standard Contemptor Pattern Dreadnought's armament with a twin-linked Heavy Bolter attached to one arm and a massive Dreadnought Close Combat Weapon on the other with a built-in twin-linked Bolter. It can also be equipped with a Multi-melta, a set of twin-linked Autocannons, a Heavy Plasma Cannon, a Kheres Pattern Assault Cannon, a twin-linked Lascannon and a power-leeching Heavy Conversion Beamer. The difference between the Osiron and a standard Contemptor was that the Osiron's Dreadnought Close Combat Weapon was always a Osiron Force Blade, and its twin-linked Bolter was loaded with Asphyx Shells. The presence of the Force Blade makes the Osiron Dreadnought the first documented Dreadnought Force Weapon and the first documented psychic Dreadnought.
It is unknown if these Dreadnoughts are still in use in the 41st/42nd Millennium. Although due to them being all psykers, it is possible that a few of them survived the Burning of Prospero and are currently nerdin out in Sortiarius.
In the Horus Heresy tabletop, the Osiron Dreadnought are your Elite Contemptors which can unload a psychic shitstorm. These precursors to the 40k Librarian Dreadnought cost around an extra 50 points, and gives each dread ML1 (with access to Telekinesis, Telepathy, Pyromancy, and Divination), Adamantium Will, and a snazzy new Force Weapon and Asphyx Shells. The Force Blade itself is a Strength x2, AP 2 monstrous Force Weapon. Have fun dicing TEQs for breakfast.