To kill Marrow you will need a weapon with Westernesse or Belariand damage. When you pull him, any nearby adds will aggro to you regardless of your level. Try to get him on a stairwell or somewhere by himself.

Marrow is one of the six named needed for the "Executioner of the Wicked" deed in Haudh Iarchith.

Here is a map for the rest of the "Executioner of The Wicked" deed