it’s mentioned somewhere in the Forum that you get a Myth Stone that is chosen between the 2 rarest Mythstones used to make it.
farming floor 500ish gives the best chance for dropping Ultra Rare Myth stones, especially when killing Epic+ monsters. Mythical Set also increases the chances of Mythstones dropping.
or Farm for lots of Gold and upgrade lower Tier Myth Stones to higher Tier Myth Stones. the trade off is 2 lower Tier for 1 next higher Tier Myth Stone. or spend 5 millionish Gold in the Shop when Ultra Rare MS & CS show up.