The rule was that a god simply can't enter creation, not only is it forbidden but its not possible. The real breakthrough was made by Trenton during the Age of Magic. He discovered a ritual which would allow a god to enter creation and rushed back to his people to use it to save his people who were dying. This was one of the D&D campaigns I won, and it went over many rl years. By the end he stood at his homeland with the ritual in hand and his people on the verge of death. But he knew that summoning Danalin would save his people but restart the godswar. After all of his traveling and friends met amoung many races (and the realization that many more would die if Danalin was summoned) he decided not to use the ritual. The Aifons were destroyed and Danalin, crushed and angered by the loss of his people, slipped into a deep sleep. Much later that same rituals was discovered by the Illians. But they didn't share Trenton's compassion and decided to risk the godswar and summon Mulcarn into the world.