Here's that heavy main riff to The Ar by Tiamat:
Tune 1 step down (D-tuning)

Part 1 (D):
D---------------------|
A---------------------|
F---------------------|
C-2--2-2--2-----------|
G-2--2-2--2-0-3-2-----|
D-0--0-0--0-------3-5-|

Part 2 (F):
D---------------------|
A---------------------|
F---------------------|
C-5--5-5--5-----------|
G-5--5-5--5-3-6-5-----|
D-3--3-3--3-------6-8-|

The song starts with the end of Part 2:
D-----------|
A-----------|
F-----------|
C-----------|
G-3-6-5-----|
D-------6-8-|

The verses are really just keyboard and bass switching from D to F.