A seclusium, according to this book, is a wizard’s study, a magician’s lab, the place where a mage goes to study his craft. But the most interesting are in the “vulnerable phase” and feature portals and magical spaces, the remains of dangerous experiments, unrestrained former guardians, and fantastic treasures.
The Seclusium of Orphone of the Three Visions is both a guide to the seclusiums of three individual wizards and a handbook for the design and use of seclusiums in any campaign. It includes rules, guidelines, and 50 pages of lists.
The book is written for Lamentations of the Flame Princess but should be usable in other traditional fantasy roleplaying games.