I'm currently lvl 38 SMN and every time I get a new spell/skill I get thrown for a loop. So could someone explain where/if Ruin II and Fester fit in?

Luckily I'm experimenting out the field instead of a dungeon, but the best I can figure is that Ruin II is a mana-eater if spammed.

And I'm not quite sure I understand Fester. Is it a 100 potency/DOT ATTACK or a buff to existing DOTs? The wording is a bit confusing to me.