Black magic is the use of supernatural powers to fulfil selfish purposes. Black magic practices and rituals include true name spells that are used to control someone using their name, Immortality rituals, necromancy and curses. Black magic is not always bad, it is simply a magic which works to do something to change the situation. It is based on the intention that you put into a spell that makes it good or bad.
In every spiritual practice, there are people who take advantage of these black magic spells to harm others. These are manipulative powers to influence others. While performing black magic spells, you should keep in mind that what you send into the universe will get back to you, if you send something dark for your own benefit then it will be back to you with the same energy after sometime. This does not depend on the spells, it mainly depends on the intention and purpose behind every magic. The magic is not black but intentions can be.
The main thing which you have to keep in mind is that that intention of the person casting the spells determines whether the magic is black or white. Black magic is generally performed to hurt someone to obtain what you want using spell casting. Black magic spells are mainly powerful projection of the intention. With the increase in the number of people casting the spells, chances of its being successful will also increase. It is not mandatory to use candles and incense to cast spells, but the casting should come from your heart and makes your intention stronger.
You can also reverse the effect of black magic spells using different methods to reverse them like casting white magic spell on them. If you are using black magic spells on someone with high frequency of love, joy an compassion then the person is unlikely to be affected by the intentions of dark spell casters.
