Right upon its release, Arven is likely to be played in Miraidon ex (which plays both Forest Seal Stone and a powerful new Item card Ill mention later) and in builds of Gardevoir ex that run Sky Seal Stone
Since they do not have any removal spells, if you can start the game early with multiple one drops and/or an Eidolon of the Great Revel with removal up, its difficult for them to make their way into the game
This is the Cardboard Carapace of black-bordered MTG
If Scourge is in the graveyard, you can get up to 30 power back of creatures with Nethrois enter the battlefield ability which can be powerful if you have big threats to return
This makes it useful when you not only want to disrupt, but draw deeper into your own deck
(They attack each combat if able and attack a player other than you if able.) Whenever a goaded attacking or blocking creature dies, you create a Treasure token