Metamorphosis — ruling 1

You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.

Metamorphosis — ruling 2

Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.

Metamorphosis — ruling 3

This mana may be used on additional costs to cast the spell, such as Kicker.

Metamorphosis : règles, fonctionnement et interactions (MTG)

Metamorphosis 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

As an additional cost to cast this spell, sacrifice a creature. Add X mana of any one color, where X is 1 plus the sacrificed creature's mana value. Spend this mana only to cast creature spells.

How it works : Metamorphosis

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
This mana may be used on additional costs to cast the spell, such as Kicker.
No other card is named in this ruling.