The Grim Reaper is an Undead Reaper boss, that was brought up from Hell to work for Jolly Roger. Grim Reaper brought up undead enemies though, and Jolly Roger didn't like that he was doing this. He banished him to Tormenta. He left the island on the Diablo. He's the summoneer of the ship. In other words he brought undead souls to work on the ship. He is a powerful boss. He's lv 59 to be close to lv 60.
He only drops his most powerful ability if you can defeat him, the power to bring back the dead. Needless to say, nobody has defeated him.
He is only on the Diablo ghost ship, looking for undead souls to take to the other side.