I have had a Necromancy Sigil the Demon Arielis with me since i was in the 3rd grade. I have a witch blood line, and i practice simple forms of spells and magic. I do plan to study Necromancy when im older since it's a gift that seems to run naturally with my bloodline. I do not know of anyone that knows of this demon.. And i havent ever seen his sigil.. So i have drawn the sigil of him since i was in the 3rd grade because it came to me somehow. I had to ask my Aunt who is part of my bloodline if she knew what the sigil meant for Arielis, so she had her friend who specialized in Necromancy and found the sigil in the lost books of Moses. He was actually sent against me a couple of months ago for an injustice reason.. And the friend of my aunt described the demon to be just like what the sigil was aligned with. My true and main question is why has this sigil been with me since 3rd grade? And i know a demon wouldn't give its personal sigil to a kid..
Well, it seems that printings of the seal of Arielis are fairly simple to come by. Nowadays anyways. Heck I found giftcards with it printed on them in Amazon. Based off of some light research it seems to be a seal to compel obedience in others. At least that's the most basic description.
Most likely, it is a form of protection through representing personal authority over negative or outside influences. So in drawing or using the sigil it is an expression of will against outside negative influence. The name Arielis itself seems interesting, because it seems to constantly refer to a shorter name of Ariel, which is sometimes seen as a demon, but is also referred to as an angel, and can be compared to the Christian Uriel.
based off of what I've learned so far, it seems the sigil might have been given to you as a measure of protection. If your family has a history of connecting with magic and practicing in more left-hand path attitudes then it would make sense for your family to want to empower you with a way to command away negative influences to keep yourself safe. or they may have empowered the seal themselves to be a force of authority in claiming you as protected.
In answer to your question on why the sigil has been with you is simple, You've had to have seen it somewhere. Studying and reading is a very good and wise thing to do, and you say you have a witch blood line, so I would ask one of your blood line descendants to mentor you in the crafts. Bloodline/ family magick is strong magick.
As for true Necromancy, it takes many years of studying. Necromancy is the ability to commune and control spirits of the higher realm as well as the underworld. It is very hard to find actual methods and historical readings that are not fiction due to the guard of the Roman Catholic Church. My Father was a Necromancer for many years and was associated with the Roman Catholic Church ( The Vatican and private Library ). He would summon demonic beings and perform exorcism. One lesson I learned from him was to never summon a demonic being especially the counter part of the high arches to do your bidding!! When the deed is done they do come back for a payment. Their payments are not like an offering to a God or Goddess with Rum or cakes. They hit you hard. My Father had years stripped from him in a matter of two days. Black hair went white and skin aged a decade. My payment landed me in and out of the hospital for 7 years. I was in my 20's health and very in shape. After I summoned the 7 I paid a hell of a price. ICU, Life support, heart failure, and so on. The Dr's. couldn't figure it out and had no explanation's as to why. I no longer practice. The closest I get to Necromancy is communing with spirits and recognizing it in full form.