Nighthowler — ruling 1

Nighthowler's last ability functions only from the battlefield. In other zones, Nighthowler is a 0/0 enchantment creature.

Nighthowler — ruling 2

On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.

Nighthowler — ruling 3

Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.

Nighthowler — ruling 4

Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its {T} abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.

Nighthowler — ruling 5

If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.

Nighthowler — ruling 6

Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.

Nighthowler : règles, fonctionnement et interactions (MTG)

Nighthowler 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

Bestow (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached.) This creature and enchanted creature each get +X/+X, where X is the number of creature cards in all graveyards.

How it works : Nighthowler

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
Nighthowler's last ability functions only from the battlefield. In other zones, Nighthowler is a 0/0 enchantment creature.
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.
No other card is named in this ruling.
#3September 15, 2013· Wizards of the Coast
Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.
No other card is named in this ruling.
#4September 15, 2013· Wizards of the Coast
Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.
No other card is named in this ruling.
#5September 15, 2013· Wizards of the Coast
If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.
No other card is named in this ruling.
#6September 15, 2013· Wizards of the Coast
Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.
No other card is named in this ruling.