Shadrak Meduson's ultimate fate is currently in question, as his identity was at some point taken by none other than Alpharius, Primarch of the Alpha Legion himself. Assuming the mantle of Meduson and garbing a unit of his own marines in Iron Hand battlegear, Alpharius' disguise was completely undetectable by the members of the Iron Hands legion he met. Travelling in a vessel that appeared to be the Iron Heart, Meduson's own flagship, Alpharius fell in with a seperate unit of mixed Istvaan survivor marines under the command of Cadmus Tyro and convinced them, in his guise as Meduson, to aid him in an action against other members of the Alpha Legion. His deception was revealed towards the end of the affair, and the survivors of Tyro's force escaped with the knowledge that Alpharius had been impersonating Meduson.
As to whether Shadrak Meduson still lives, or whether he was killed by Alpharius, or Tybalt Marr, or any other hazard to a loyalist marine operating behind enemy lines during the Horus Heresy...that is currently not yet known."
-via Lexicanum (http://wh40k.lexicanum.com/wiki/Shadrak_Meduson)
"Following the Drop Site Massacre, a mentally volatile Fulgrim would have Fabius Bile clone Ferrus. However these clones proved unstable and each refused to turn against the Emperor, forcing Fulgrim to once again kill his brother and driving him even madder."
-via Lexicanum (http://wh40k.lexicanum.com/wiki/Ferrus_Manus)
Shadrak Meduson's amalgamation of Iron Hands, Salamanders, and Raven Guard continued their campaigns against the Traitors for long into the Horus Heresy, until he disappeared. His preferred targets were those legions who's betrayal stung the most: Emperor's Children, Sons of Horus, and Alpha Legion. Utilizing new tactics learned from collected survivors of the Raven Guard and Salamanders Legions, Meduson staged multiple raids and lightning offensives, aimed at severing the leadership of the Traitor legions.
However, after the Battle of Dwell, sightings of the Shattered Legions Warleader became sparse, conflicting, and sometimes outright contradictory. Who is to say what really happened to Meduson? I imagine we will find out in upcoming publications from the Black Library.
In the meantime, I propose this alternate backstory. Mostly it is to support my vision of my 30k forces. I hope you enjoy it...
After the failure of the assassination attempt on the Sons of Horus leadership, the decision was made to split the fleet into smaller raiding parties. "We present an easier target, the larger our force collected under this banner," Meduson is quoted to have said. Taking to heart the new tactics he had learned from his Salamander and Raven Guard advisors, the decision was made to split the might of the Shattered Legions, to detract attention and give credibility to the rumors that the Warleader was indeed slain.
An assassination attempt would not be successful at this point. The failure of the previous attempt meant that the guard against such a coup d'etat would be vigilant, indeed. So secondary targets were assessed, and the Shattered Legions waited, and watched.
Their careful observation was shattered when Raven Guard observers discovered a horrifying realization. They had discovered that the genetic material of the fallen Primarch Ferrus Manus was being transported by the Emperor's Children, presumably so their foremost apothecary (Fabius Bile) could begin his foul experimentation on it.
Coupled with the secret information stolen at the Battle of Dwell, this genetic material could give the Traitors the means to create their own Primarchs.
Disregarding earlier agreements to disperse the collected Loyalist forces, Meduson rapidly assembled a strike force and set off after the Emperor's Children fleet, intent on securing the genetic material of his Legion's sire. This was both an emotional and logical goal. Letting the Traitors to profane the corpse of his sire was a personal insult. Furthermore, allowing the Traitors to unlock the process behind the creation of the Primarchs would be disastrous for the Loyalists arrayed against them.
The speed in which Meduson made his decision, collected his forces, and departed did not allow for tactical data to be forwarded to his field commanders. Alone, save for the forces accompanying him, he made the utmost speed for the last known location of the Traitor fleet bearing the remains of the Gorgon.