"...If players are not attached to Stoic's character, the reaction will probably be "Why should I care about this dude..."
True, I probably wouldn't have cared too much about him if the whole flashback was at the game start. I was not thinking of a flashback though but more as starting the game when Stoic was human (Zeno Slade - I think?), so we were introduced to how he became a skeleton at the very begining, he was quite a likeable character with his high ideals of how the world should be and his dislike for racism, beingism, udeadism, er different inteligent beings not getting along. Then Arc 2 would be the battle with the vampire start we now have. Now I know the whole story (up this point) then the method you used was quite good (flashbacks towards the end), like I said, just a thought, either way would be good.