Constellation — Whenever Agent of Erebos or another enchantment enters the battlefield under your control, exile all cards from target player's graveyard.
4/26/2014 The constellation ability is mandatory. If you are the only legal target (perhaps because your opponent has hexproof), you must target yourself and exile all the cards from your graveyard.
4/26/2014 A constellation ability triggers whenever an enchantment enters the battlefield under your control for any reason. Enchantments with other card types, such as enchantment creatures, will also cause constellation abilities to trigger.
4/26/2014 An Aura spell without bestow that has an illegal target when it tries to resolve won’t resolve and put into its owner’s graveyard. It won’t enter the battlefield and constellation abilities won’t trigger. An Aura spell with bestow won’t be countered this way. It will revert to being an enchantment creature and resolve, entering the battlefield and triggering constellation abilities.
4/26/2014 When an enchantment enters the battlefield under your control, each constellation ability of permanents you control will trigger. You can put these abilities on the stack in any order. The last ability you put on the stack will be the first one that resolves.