The Familar Retreat is an organization dedicated to storing familiars and keeping them safe. When interacted, you will be greeted with a black being: a Familiar Representative. The retreat wil be unlocked after finishing all the trials at the Temple of Trials .
The retreat keeps familiars and releases them to their owners, should they want to use them again. You can store up to 400 familiars.
After certain key events, like defeating Shadar, the familiar representative will give the player items to aid them in their quest.
Retreats can be found in most towns, and in almost all dungeons. Familiar retreats are often found near waystones in dungeons or near The Cat's Cradles in towns.
In the Wizard's companion, there is a statement that suggests that the Familiar Representative and The Familiar Retreat are in fact of another world, thus explaining why so many familiars can be kept and why they appear in even the most dangerous and unexplored places.