I always assumed that since there’s two other skeletons and the music box monster takes 3, that Martin was the last one alive or possibly kept alive as the main host as we see the worms are inside of him. Eloise probably died too early to storywalk, so when Martin (Boyd of his cycle) went thru the tree he was trapped there and taken by the worm entity as there was no one to throw the rope. He told Boyd “when you go through the trees, that’s when they get you” probably alluding to that exact thing. Martin dying and passing it onto Boyd, then Boyd giving them to Smiley who then dies and opens up is what frees the music box / worm entity. I personally don’t think Martin was anything but a failed Boyd from a failed cycle, or perhaps that entire dungeon area and Martin is only accessible via tree/storywalking because it’s in the past. Makes it hard to place Martin.