I finished Arniel's Endeavors and got "Summon Arniel's Shade".

But there seems to be something wrong with it. It's just too good.

It costs me 0 magicka and he does not seem to have any health.

If I encounter a enemy that is too dangerous, I just let him loose and he battles the enemy for 60 seconds. Then, I can let him go again at no cost.

Is this a known issue? It almost feels like cheating using him.

And is the quest over, or am I suppose to use him later in the game?