Veiled Serpent — ruling 1

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Veiled Serpent — ruling 2

When it turns into a creature, it is no longer an enchantment.

Veiled Serpent — ruling 3

It changes into a creature even if the spell is countered.

Veiled Serpent — ruling 4

It triggers when the spell is cast, which means it becomes a creature before that spell resolves.

Veiled Serpent : règles, fonctionnement et interactions (MTG)

Veiled Serpent 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

When an opponent casts a spell, if this permanent is an enchantment, it becomes a 4/4 Serpent creature with "This creature can't attack unless defending player controls an Island." Cycling (, Discard this card: Draw a card.)

How it works : Veiled Serpent

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
When it turns into a creature, it is no longer an enchantment.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
It changes into a creature even if the spell is countered.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
No other card is named in this ruling.