All the gurus out there, and anyone who knows who Gallowbraid and Morinfen are, please give me a detailed explanation on who, and what they are, and how they fit in the story line.

~Myrddraal

Here's what I know. This takes place during Weatherlight, after Sisay convinces Mirri, Rofellos, and Gerrard to join.

They are two (brothers?) from Rath who were sent to attack Crovax's mansion. The Weatherlight crew reached there and fought them. During the fight, Gallowbraid killed Rofellos, and they were then either killed or escaped back to Rath. Gerrard, enraged by the death of his friend, left the Weatherlight with Mirri, and then he split paths with Mirri - Mirri going to Llanowar and Gerrard going to Benalia.

These two may have also attacked Multani during the Weatherlight story, but I am not sure if it was them or some other Phyrexian forces.

G&M
 

They're both like me, agents of Y@vv9nn0+|-|. They're brothes as stated by Zazdor and generals of some sort from Our Holy Ground, Phyrexia.
They and a small army of phyrexians attacked Urborg Mansion as a first step by our Holy Father to convert Crovax.
Gallowbraid slew Rofellos only to be killed by the Angel, Selenia. Seeing the death of his brother, Morinfen leapt and attacked Selenia, but unfortunately, he too was killed.
Hope that helps...

Long ago, Sisay, Gerrard, and the rest of the Weatherlight crew had brought Crovax home to his family estate to do battle with two of Volrath's evil servants, Gallowbraid and Morinfen, who were bent on the destruction of Crovax's entire houshold for reasons only known to Volrath. The ship departed once Crovax summoned Selenia - a guardian angel contained within an artifact marked by an ancient sigil - and defeated Volrath's servants.

As the Weatherlight returns in the Urborg Swamps they find Crovax's family destroyed by the forces of evil, which had returned in greater force to lay siege to the estate in Crovax's absence. They then join Crovax in confrontation with Gallowbraid and Morinfen. After the defeat of the evil ones Crovax elects to join them.