au: the seven archangels in the inception verse
An idea can always exist so long as someone believes in it, and that counts beings. If the only place they exist is in a person’s mind, they get trapped there, left with minimal tools to get out. Castiel did not only see Lucifer, but all of his brothers, jailed within him, all wanting a way to get free. To do that they find they have to force themselves to work together to make Castiel believe they can leave his mind and be in a human vessel again. However with each angel having very different intentions, things can become even more complicated than they already are, and ideas are not idle play things.