I have collected this intro through experience and tutoring with friends, I find this
fairly simple. It's a great song and it sound awesome once you start to play it!
[ Tab from: http://www.guitaretab.com/b/black-sabbath/348385.html ]
e:--------------------------------------|
B:--------------------7--9--------7--9--|
G:-------------7--9---------7--9--------|
D:---9--9--9----------------------------| x4 (play with heavy distortion for a greater sound)
A:---7--7--7----------------------------|
E:--------------------------------------|
The song goes on to continue with the guitar and bass playing the same riff.
This was my first tab and I hope this helped.