I read 1 book from this series back in 1996. It was book 3 in the series;however, I don’t know just how many books the series had.
It starts off with the main character sitting in a chair in front of a fire place, giving an outline of his adventures as a child, including referring to himself or his friend as a key, or the key, or something like that.
The story then shifts to when the main character was a kid. Him and the friend go to this haunted house, hunting for why it was haunted, when they run into some other kids. Weird stuff happens, they find a crystal skull, and get sucked into the astral plane, where they are attacked by little creatures that smell, and maybe taste, like spam.
The main character can understand these creatures, and he transforms them into weapons for all the kids. At one point the only female character meets her grandma in the astral plane, who has been dead for awhile. Both of them are native americans.
Later on, the kids end up at a castle, where they find the school bully, who had disappeared in the house earlier on. A battle ensues, and it’s revealed that the bully is being transformed into an Old One. During one part of the battle, the main character realizes he can see a cloak flickering around his friend, which had been absorbed into the friend in a previous book. He gets the girl to release the friend’s soul, which allows the friend to tap into all the powers he’ll have in the future.
The kids win the battle, return to earth, and narrowly escape the house before it collapses.
If it helps any, I received it as part of my Fear Street book club subscription in 96, so if anyone else was a member, maybe they got it too.