Here's the first card with Amass ability in this list. Basically it puts a 1/1 zombie token onto the battlefield alongside the 2/2 creature with flying. If you already have a Zombie Army token, then it will simply add a +1/+1 counter to it.
This means that Aven Eternal is good in all types of decks regardless of whether they are built around Amass ability or not. So this definitely makes it one of the best blue commons in War of the Spark.