Manaphy

Fly to the Ruins of Void’s entrance. Then use Rock Climb to scale the wall nearby.

Walk west after climbing the wall and use the second Rock Climb wall to descend to the area west of the ruin’s entrance.

Use Surf to go to the Dive spot here, then use the ADM to go underwater.

Go east and enter the first passage to the north.

Follow the path north after entering the area.

Go through the first passage near the end of the path forward.

Continue following the linear path forward before entering the final passageway at the end.

You’ll see a Toxapex at the end of this area. Interact with it to initiate a Dynamax battle.

Defeat Toxapex, then backtrack to the previous area.

Reach the area you initially descended from. But this time, instead of going west where you came from, follow the narrow path east.

You’ll enter a slightly open area, but instead of ascending here, just follow the narrow path north.

Continue north and go through a passageway.

If a tablet is blocking the path then that means you didn’t beat the Dynamax Toxapex.

The area behind the tablet has a Manaphy egg that you can pick up and hatch to obtain the legendary Pokemon.