The Rome Statute is the governing document which sets out the existing Crimes Against Peace. Ecocide was to be included in earlier drafts, until it was removed in 1996. You can read about the history of the law of Ecocide, which dates back to 1972 in The Human Rights Consortium research paper, called Ecocide is the missing 5th Crime Against Peace.
The amendment submitted to the United Nations in March 2010 set out the legal reasoning for a law of Ecocide and the definition. Polly Higgins proposed that Ecocide be legally defined as “the extensive damage to, destruction of or loss of ecosystem(s) of a given territory, whether by human agency or by other causes, to such an extent that peaceful enjoyment by the inhabitants of that territory has been or will be severely diminished.”
You can read the full written proposal in chapters 5 & 6 of Eradicating Ecocide: laws and governance to prevent the destruction of our planet by Polly Higgins.