Sunderflame is a medium armor set. When you deal damage with a fully-charged Heavy Attack, you deal an additional 4000 Flame Damage and apply Minor Breach and Minor Fracture, reducing the enemies' Physical and Spell Resistance by 1320.
The Sunderflame set is part of the base game. It drops in City of Ash I and City of Ash II in Greenshade. Each set item is bound on pickup. You can acquire medium armor, weapons and jewelry of this set in the Daedric motif style.