"写神话" can be translated into English as "write myths" or "compose myths", depending on the context.
"Write myths" is straightforward and commonly used when referring to the act of creating or documenting mythological stories.
"Compose myths" carries a slightly more artistic or creative connotation, suggesting the deliberate crafting of these stories.
Both expressions are accurate, and the choice between them may depend on the nuance you wish to convey.