Dark King

The Dark King is the final boss of Final Fantasy Mystic Quest. He initially takes a humanoid form, but later grows extra arms, each carrying an enchanted weapon. He then reveals his true form, that of a giant spider, and eventually transforms into a squid-like version of the giant spider before he is finally defeated. The Dark King's attacks vary depending on what form he is in. In his original form, he is not very powerful and will attack with his cane. In his multi-arm form, he wields weapons like the Quake Axe. In his spider form he can use Golden Web which petrifies whomever it hits as well as spider kids. In his final form, he wields the spells Mega-White and Mega-Flare, more powerful versions of the best two offensive spells to be used on him.

Although the best "normal" way to defeat the Dark King is by using Flare and White on him, the Dark King is even more vulnerable to Cure (perhaps he's undead or something).