The Twelfth Astrological House - Evil Spirit
The Twelfth House: Also known as "the Evil Spirit".  In contrast to the opposite house which is called the "Evil Fortune", this place is often associated with the mental (rather than physical) pain and suffering induced by spiritual issues. According to the ancients, Saturn rejoices in this place. This is probably because it likes to suppress the knowledge of what is beyond its own bounds. Paulus states: "When the star of Kronos [Saturn] occupies a masculine zoidion [sign] for a diurnal birth, it alone rejoices in this place".  This is a condition known as hayz, an ancient form of dignity that takes into account the sect of the chart, but it was not considered quite as favorable there at night. Even so, Saturn naturally has the easiest time rejoicing here because this is the last place we get to in the natural course of life through the 12 archetypes. It is also the slowest of the visible planets and the reason why it comes in last place. All of this is very symbolic of the cause of any spiritual pain or madness: agnoia. Saturns color is black, which is highly symbolic of darkness and ignorance. In other words, not knowing the truth will not set you free. Astrologers say that this place gives signs for retirement, restriction, imprisonment (not necessarily physical), mental confusion (the extreme being an insane asylum), secret enemies, living abroad (in the sense of exile), self undoing (in the sense of guilt or being haunted by demons or ghosts from the past), taboos, hangups (in the sense of what makes us uncomfortable), procrastination, and neglect, slaves (as in their character) and quadrupeds, etc ... This place is also naturally hidden. It therefore represents secrets, skeletons in the closet that the native does not want to come to light, and because it is representative of spiritual issues, also the unconscious and subconscious mind. Frequently people with a heavy emphasis of planetary alignments here can be seen to deal with the poor and less fortunate, through monastaries, mental institutions or relief agencies like the Red Cross that deal with unforseen incidents.
The 12th sign from the ascendant is often considered to be the garbage bin of the zodiac. This is because it represents what is left over from the previous 11 houses (misfits), and therefore, what is left undone. Some astrologers believe that this place represents the most recent previous incarnation. Along with this idea, therefore, they believe that it represents the reason for the current incarnation, caused by the souls desire to rectify unfinished business from the last lifetime. The idea of neglect is a key ingredient here, because in order to complete and balance out the circle of existence, we need to confront all of these issues, a few of which will seem unpleasant. Those things that are, will naturally be avoided until the pressure to deal with the issue becomes too great to ignore. Closely tied into this idea is the astrological concept of aversion; which is any two signs that do not have a Ptolemaic aspect between them. The 12th house is in aversion to the 1st house by nature, and the 1st house is what we immediately pay attention to. An aversion to immediate attention to any issue is neglect, procrastionation, etc...
The karmic school of astrology makes heavy use of this place along with two other key factors in the chart: the Moon and the nodal axis. As I have already mentioned, the 12th represents the immediate past incarnation. In this approach, the south node represents those qualities that we have an excess of (over the course of many lifetimes), and the north node, those qualities which we tend to have neglected in the past. The north node and the 12th house therefore have this in common as a driving force for the purpose of this lifetime and how we plan to finish or balance spiritual growth. The squares to the nodes represent distractions, or potential traps on the way to the goal. The Moon represents the past and habits in a more general sense or it can also represent what we need for the future if its qualities are more in agreement with the qualities of the north node. These 3 positions represent a major "molecule" from which astrologers can ascertain much about the "compounds" behavior (souls development and future direction of growth) and hopefully help triangulate their approximate position.
More info on each of the planets that have dealings with this place:
71. Paulus Alexandrinus, Hellenistic astrology Introductory Matters 378 A.D. © 1993 Robert Schmidt, through Project Hindsight, Published by The Golden Hind Press. See pg. 60. Schmidt now calls "divinity" spirit instead.
74. Vettius Valens. Anthology, Bk II, Part 1. aprox. 160 A.D. © 1994 Robert Schmidt, through Project Hindsight, Published by The Golden Hind Press. See pg. 10.
76. Paulus Alexandrinus. Introductory Matters 378 A.D. © 1993 Robert Schmidt, through Project Hindsight, Published by The Golden Hind Press. See pg. 60.