How to sort by arbitrary keywords?

How to sort by arbitrary keywords?

I have the following query:
SELECT
*
FROM
`Magic The Gathering`
WHERE
`set` = 'Magic 2013'
ORDER BY
`rarity` ASC
LIMIT
500
Rarity consists of the following keywords: Mythic, Rare, Uncommon, Common
Currently it sorts alphbetically, so it's sorted as Common, Mythic, Rare,
Uncommon.
How can I sort rarity so it's displayed in the following order?
Mythic, Rare, Uncommon, Common