Is Practicing Tantric/Wrathful Meditation Dangerous Without a Guru?

Some time ago, I found a mala in a shop that I was quite drawn to for reasons I cannot explain. At the time I thought it was wood but as the stain wore off I saw it was bone. This lead me to do some research into Bone Malas and I learned about tantric buddhism and wrathful meditation. This in turn lead me to a mantra I started repeating with the mala 108 times every day. Everything I read said I needed a master or guru to help initiate or guide me. So my question is how dangerous is this? Do I need a guru, and how on earth does one get initiated into a a type of buddhism?