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My friends and I were staying at her house which is in the 'ghettos' as some might put it and in her basement there were about four spirits down there one we could see as clear as day on, the angry one, stayed hidden as well as the little boy(Aaron) and the little girl( Juileanne) well the angry one possesed my friend twice and tried killing my boyfriend, me he didn't have a problem until I tried to help my boy friend. then he was trying to possese me, i felt like my insides ere being cut by a teethed knife. her got into my head he really didn't like my bf.

Have you since invested in protective measures?

You could take any numbers of protective measures. Here are a few recommendations and options for you if I may.

Setting wards or boundaries, to ensure the sanctity of the rest of the house.

Creating a keepsake to ward against negative energies.

Work on emitting sincerely positive energies to counteract any negative energies that may be creating an unbalance in the surrounding.

Spiritual grounding can also be a very positive tool in said situation.

Various rituals may help in said situation as well.

There are many different options, the difficult part may be finding the appropriate one for the situation. Hopefully one of these recommendations may help, or will direct you to one which does.

In the vein of Garrett's suggestion of wards or boundaries. I have a Fortress Spell that my friend and I created to set up a permanent protection for her grandfather. Basically we used witch jars, filled them with protective herbs, stones, specifically ones that ward off negativity, we used oils, black wax, salt, nails etc. We charged them then buried them around his house in the four cardinal directions, and charged a second time, visualizing a medieval castle fortress, a huge impenetrable stone wall around the house, blocking anything negative and deflecting it. We have had it up for around 2 years now and it has worked very well so far.
Since we were burying them we used glass jars. (baby food jars to be exact since at the time I had at the time. )

I would imagine so. We chose this method because it was her grandfathers house and at the time we didn't know how long either of us was going to remain in the area, so it was important we use something that would remain without maintenance or recharging.

I have not tried that, it sounds like a good idea though. I don't see why the life energy wouldn't contribute, since most the associations for flowers and plants are based off their properties, which would be present with or without charging. I think trees might work best for that purpose because they tend to live longer than most flowers or plants, at least in the chilly state I am from.