Nether Shadow

Say you're the owner of both a permanent and an Aura that's attached to it. If both the permanent and the Aura are destroyed at the same time (by Akroma's Vengeance, for example), you decide the order they're put into your graveyard. If just the enchanted permanent is destroyed, it's put into your graveyard first. Then, after state-based actions are checked, the Aura (which is no longer attached to anything) is put into your graveyard on top of it.

Note that bringing the Shadow back onto the battlefield from the graveyard is not a spell, it is an ability. It can't be countered with something that counters spells.

Since it enters due to triggering at the beginning of upkeep, it is not possible to get an infinite loop with four Nether Shadows.