The combination of four or more planets in one sign(rashi) in a horoscope is suggested to cause 'Sanyasa Yoga,' in Vedic Astrology, which is the combination for renunciation, unless there are combust planets in this combination. Combustion of planets happens if they are adjacent to Sun, which means that they're placed with in the 0 to 15 degrees on either side of Sun. Some experts are of opinion that Mercury is never combust.There seems to be some problem with this combination in the sense that I do come across many charts with it but not all of the natives are Sanyasins. The charts however, more often than not, suggest the natives with a great focus in a certain field in their lives.
There are twelve houses in horoscope. These houses represent everything possible in every area of life because one house represents many matters. For example: The second house tells about language, mouth, family, wealth (accumulated) and so on. This means that many planets getting accumulated in a horoscope indicate that the native has a lot of Karma related to that house. It's true that every planet casts seventh full aspect to the house seventh from the one where it's situated. These aspects are considered only and only in the Lagna chart and not in the Bhava chart. Taking into consideration this fact, you would realize that stelliums in the charts suggest that such natives have two highly pronounced domains in their lives. However, it's certainly a very easy task to ascertain which matters related to the life are clearly indicated, unless you clearly analyse all other combinations and divisional charts. The Karmic baggage in the lives of the natives with Stelliums is more perspicuous if their Bhava-charts are not much different from Lagna-charts.
For example: If in a native's horoscope, eighth house is highly populated, it means that the matters pertaining to the eighth as well as the second (seventh from the eighth) would be considerably highlighted. Secondary importance would be of the houses of which the occupants of the eighth house are lords and the houses where the tenants of eighth house cast their additional aspects upon.
Eighth house signifies death, longevity, inheritance, self-transformation, sudden changes, mysterious and occult, scandals and so and so on. The second house represents speech, languages, family, teeth, food and eating habits, accumulation of wealth, face etc.
If there is a Stellium in the eighth house--it means that these matters would be most pronounced in the life of the individual. The natives with stelliums in their charts don't tend to live balanced social lives. They tend to be hyper-functional in certain domains and dysfunctional in others.
Stelliums in dusthanas, which is in the sixth, eighth and twelfth houses are most difficult, whereas in the first or the tenth house they cause a person with great fame and achievements. Since tenth house is best of the houses, the stelliums in the tenth house causes great Rajyoga if other combinations support.
The Stellium of planets strengthens all the planets involved unless they're very inimical to each other in nature. For example: Sun and Saturn involved in any conjunction cause natives to have stringent relations with authority figures like father since their childhood.
Stelliums are easy indicators of Karmic baggage. For example: A person with a Stellium in the tenth house has played commanding roles in society and has earned distinctions in his earlier incarnations and hence he comes up in life in a most dignified manner. A chart with a Stellium in the sixth house, on the other hand, might suggest a Karmic script of debts, prisons, arguments and enemies.