Shadowchild came from the corpse of a white bird. What form does He-Is sometimes take, again? A white kingfisher? Just a little something to think about, a possible parallel or connection between Shadowchild and Sweetgrass-Voice.
Read the last panel here — the skin lizards say that, when people die, their hearts became kingfishers. And when Ed dies, a kingfisher appears out of nowhere...it's probably He-Is, but it is also possible that it could be Ed. That would add multiple new layers of Tear Jerker to Digger's terse treatment of it if true, though...
It's strongly implied that Sweetgrass-Voice managed to Show up at the monastery by stretching himself thin, the same way shadowchild does. It's also implied that the monastery is hundreds, if not thousands, of miles away from Sweetgrass-Voice's resting place. Think for a moment on how utterly massive Sweetgrass-Voice has to be in order to stretch that far and still retain a perfectly coherent form.
The 'stretching' is implied to be more of a metaphysical than physical maneuver. Sweetgrass-Voice says "But now you have come here, where reaching is easier" when they are at the monastery. The barriers between worlds are weak enough that Descending-Helix can manifest, and weak enough that Sweetgrass-Voice can 'stretch' himself there much easier.