Whole books can (and have!) been written about Meduson (and even a book that was about him, but turned out to be not him, but Alpha Legion infiltrators -- see the Seventh Serpent from the Black Library).
Post betrayal, Meduson took command of the shattered legions and led them to righteous retribution over the traitor legions, all the while hunted by Tybalt Marr and his Emperor's Children allies. He is so good at command that he even made an attempt on the lives of Horus, Mortarion and Fulgrim.
Meduson is a Praetor of the Iron Hands and their leader, after Ferrus Manus died at the hands of Fulgrim. As a Praetor, he is a reasonable quality one with several nice pieces of kit, including a rending S+1 AP3 blade and an archeotech pistol.
However, what really sets him apart is his Master of the Shattered Legions rules. With this, he not only is an Iron Hands marine, but he also counts as a Salamander and a Raven Guard. He can use any rite of war from all three legions which makes for some solid combinations. This has come about from alloying together the disparate members of these three legions to make a force to be reckoned with, and it is a really beautiful and well fitting rule that is unique in all of the Horus Heresy.
Additionally, once per game, he can Raise the Storm as his warlord trait and give all his army with any of the three shattered legions special rules all of furious charge, hatred and crusader. This can be powerful and if timed well can be the execution stroke.
Meduson lacks an AP2 weapon and must be played accordingly. Although his rending rule make up for this somewhat, one must remember that rending is not AP2.
Having furious charge might not do much for Raven Guard members of his shattered legions force as they may already have it.
Look, you could use Meduson as an Iron Hand Praetor and build accordingly -- he is worth his points and will serve your army well, particularly with his warlord trait. But really, I would want to toy about with possible builds for a shattered legions force with Meduson as the Praetor -- very fluffy and very powerful to be clear!