Mythos of Nethroi

Destroy target nonland permanent if it's a creature or if {G}{W} was spent to cast this spell.

Magic Online Promos

"And then the shadow took living form, and the shadow devoured the man whole—body, mind, and soul."
—*Tales of the Ozolith*

Ikoria: Lair of Behemoths Promos

Ikoria: Lair of Behemoths

If an effect allows you to cast a spell without paying its mana cost, you can’t choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can’t waive a cost reduction unless that effect says you may.

If an effect copies the Mythos spell, no mana was spent to cast the copy, so the copy won’t receive the bonus.

Mythos of Nethroi can target any nonland permanent. Determine as the spell resolves whether the permanent is destroyed, either because it’s a creature at that time or because {G}{W} was spent to cast the spell.

The abilities of the Mythos check what colors of mana were spent to cast the spell. It’s not an alternative cost to cast the spell.

The ability checks what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn’t otherwise spend, but it doesn’t change what mana you spent to cast the spell.