Humanity's Last Breath, Buster Odeholm plays a 6 string tuned to drop E down an entire octave, with a .66 gauge string on the low end. Vildhjarta uses two 7 strings tuned similarly, with the .66 on the low end, they share a guitarist/producer, one of their vocalists, and I believe a bassist with Humanity's Last Breath. Buster also plays Hendrix style flipped lefty, which adds a bit of high end attack on top of it.