Sacred Haven of Debauchery

You know, most legends were created by the Church, and it interprets history to its advantage?

This isn't a Church legend. And it's not hard to figure out.

The rest, I suppose, are studying forbidden tomes, like Jowan. Only a few make deals with demons.

Most of them either consort with demons or consort with those who consort with demons. Forbidden tomes are only common in Tevinter. But there, contact with demons is no less frequent, considering the amount of undead we encounter in DA2 while opposing Danarius.