In Dark Tranquillity, he was one of only two members (the other being drummer Anders Jivarp) to maintain his role, unlike the other members who have or had switched. He also wrote some of the lyrics for the first three and recent two Dark Tranquillity albums (Lead singer Mikael Stanne wrote all of Dark Tranquillity's lyrics from Projector through Fiction) and In Flames' album The Jester Race, and continued to translate In Flames vocalist Anders Fridén's lyrics from Swedish to English for the next few albums while Anders worked to become more proficient in English. Although he quit Dark Tranquillity in 2016, it wasn't announced until March 2020.

Sundin is also the founder of Cabin Fever Media, which among other things creates artwork for metal bands. He recently published a book of independent artwork and sketches. He has done album layouts for bands such as In Flames, Eternal Tears of Sorrow, Meridian Dawn, Arch Enemy, Nightrage, Sentenced, Kryptos, and Fragments of Unbecoming, as well as his own bands.