It sounds like theyre using a 12 string guitar or an effect.
Drop C Tuning - C G C F A D
Legend
h - hammer-on
p - pull-off
[ Tab from: https://www.guitartabs.cc/tabs/i/in_flames/metaphor_tab_ver_2.html ]
D|----------------------------------6--------------|
A|-----9h10-12--10-9h10p9------9h10---12--10--12---|
F|11----------------------11-----------------------|
C|-------------------------------------------------|
G|-------------------------------------------------|
C|-------------------------------------------------|
D|--------------------------------------12-10-------------|
A|-----9h10-12--10-9h10p9----------9h10-------------------|
F|11----------------------11--9h11-----------------9h11--9|
C|--------------------------------------------9h11--------|
G|--------------------------------------------------------|
C|--------------------------------------------------------|
