Summoner's Rune Belt  

 

This is a tier 3 pet focus item, meaning summoned pets by the classes able to wear it will become noticably more powerful. The Diamond Prism of Companionship is tier 1. The Vex Thal gloves and the range item purchasable from DoN vendors for 300+ crystals are tier 2. There are more focus items (up to tier 5?) but those are the most commonly seen ones. Unless you're in a large raid guild this one is probably going to be the easiest tier 3 focus to get. Of course, I haven't gotten to do this raid yet, so I'm not sure how tough the spider is. :)