The babies. Hence why he has that big scarring on his belly. Same with Lea and Fragile. They eat the Cryptobiotes and that either gives birth to the babies OR gives them enough stuff like nutrition to keep them growing normally with Timefall making them grow fast and basically burst through the carrier as an adult. The dark enemies are adult sized babies who died and need a new baby to grow into a real person.

In the 1st trailer we see the baby disappear from Sam/Reedus' arms and then see the black hand small hand prints moving away from him.

The black things only want the babies or rather any life so they'd be "complete".

So Sam transports humans and parts of humans that keep the dark beings away. Maybe we are all Sam and we deliver babies to other players (optionally - otherwise you need to gather a ton of Cryptobiotes etc) who send a request, and we temporarily enter their gameworld and the baby and the arm mechanism shows up where the other player is or if there's a threat nearby, and that's why the lights on the arm change color, to indicate threat or friendly.