They will become upgradable to 6* as we release their respective biweekly maps, right now only the Earth Satyr Lord is available as we released the Prophet of Pestilence map.
(NOTE: the 6* Satyr Lords are designed to skill up the 2nd set of zodiac guild war bosses).
So, the pestilence prophet is ok, but really only useful if skilled up. however, to skill up to level 12, you'd need 6x11 prophets, which is 12x11 or 132 heralds. that's tough, but not the end of the world. however, you'd also need 132 rainbow maps, which is impossible. the cost of making this card useful is simply too high. i think this needs to be looked at again.