Valaran Commander is found within the Valaran fortified camp.
You can get to him via the walkway or by defeating guards and walking up a ramp.
He drops a very good Valaran Sword, a set of Valaran Elite Armour items, and a Valaran Shield.
For melee characters, this fight can be difficult. The Valaran commander fights like other humanoid enemies, but is faster, more aggressive, has much higher stats, and will attempt to parry. Obtaining a highly-damaging weapon, especially one with a slow effect, can make this fight much easier, as the commander can only attack at close range and has the same base movement speed as the player character. Parrying and counter-attacking him is recommended if possible.
For magic characters, the ice shard is recommended as it will allow the player to slow the commander's movement speed and attack him with near-impunity. Be careful not to kite him out of his arena, or his health will be restored to maximum. Instead kite him to boxes on the right side, then jump on the top box. AI of boss will crash and he will stand still, leaving him easy to kill with magic.