If the character chooses to climb into the room, they must save vs. Death or immediately fall upward, splattering onto the stone altar above! (100ft of falling damage) If they make the save, they manage to grab onto the ledge.
Indeed the portal is a magic one which actually leads to the ceiling of the Temple of Tziakohtzl, with gravity pulling in the opposite direction relative to the character's initial orientation. Hence the strange weightless sensation as the character pauses halfway through the portal.
If characters manage to descend safely into the temple, they will be welcomed by the Priests there who will revere them as "Those who have outsmarted the god". That said, they should be careful, since the priests love to play tricks themselves.