Try this out;
Way it works is since you gain a buff on yourself with either 1 or 2 stacks it's simply looking for the buff you have then displays the aura depending on what # of stacks you have. Since I have no idea what you actually wanted the aura to say for certain it just made it as an icon with it saying 1 or 2 spirits; of course you can change the aura type from icon to text and then change what it displays to your liking.
It's not anything super fancy but you don't need it to be super fancy.
*Please note that this aura will only work on PTR currently (edit - you can technically load it on live as well but it obviously won't work so if you wanted to see it in action (so to speak) try it on PTR.