Is this mandatory?!? Did you forget a "may"?

Also it should be "Aura or Equipment card".

It is more common to use a triggered ability on "Whenever ~ attacks and isn't blocked". You would have to make it a replacement effect ("If ~ would deal combat damage") to time any later, and that would also require a reflexive triggered ability ("When you do").

So long story short... I think you mean this:

> Whenever Relic Caller attacks and isn't blocked, you may sacrifice it. If you do, return target Aura or Equipment card from your graveyard to your hand.