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.
When it turns into a creature, it is no longer an enchantment.
If you have zero or negative life, half your life is zero, so it costs nothing to use the ability.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Scott Kirschner
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
