Agent of Erebos — ruling 1

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.

Agent of Erebos — ruling 2

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.

Agent of Erebos — ruling 3

An Aura spell without bestow that has an illegal target when it tries to resolve won't resolve and will be 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.

Agent of Erebos — ruling 4

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.

Agent of Erebos : règles, fonctionnement et interactions (MTG)

Agent of Erebos is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Constellation — Whenever this creature or another enchantment you control enters, exile target player's graveyard.

How it works : Agent of Erebos

Original English text — Wizards of the Coast / Scryfall

#1April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 26, 2014· Wizards of the Coast
An Aura spell without bestow that has an illegal target when it tries to resolve won't resolve and will be 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.
No other card is named in this ruling.
#4April 26, 2014· Wizards of the Coast
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.
No other card is named in this ruling.