Now, the Shadow of Rage is really just the shadow of something very dark, but it isnt the Shadow Lord, it is the Shadow of his good friend. !SPOILER!It is the shadow of Zeid who goes berserk during the last Bastokan mission, he summons them. You fight this mission with Volker, so its only natural he goes ahead and brings out the burning anger about having his friend left to die in the frozen wastes of Xabard, and THEN turned into the Shadow Lord through the evil methods of the Zilart!/SPOILER!
The Bastokan final boss, summons two of these during the 2nd half of the battle. If they are not killed fast ur tank could be hit with a Light SC which could mean death. i did this already (yay rank 10) but so hard getting the BC i say its harder getting to it (links)