My idea is that anyone with council of Esus state religion AND one shadow mana can cast a spell that doubles all diplomatic penalties against one player. For example, instead of the Bannor giving a -1 diplo penalty against the malakim for close borders, they give a -2 diplo penalty. And double all other penalties. It would have to be applied automatically each turn (maybe through the same mechanism that the constellations work). This would fit thematically with the council of Esus since it is useful, sneaky and is encouraging others to war. The shadow mana requirement is a slight but thematic nerf.