The Eidolon will not have random classes even in insane/madness

game/modules/tome/class/NPC.lua

@@ -460,7 +460,7 @@ function _M:addedToLevel(level, x, y)
 				self:learnTalent(tid, true, math.floor(lev / 2))
 			end
 			-- Give unrand bosses extra classes
-			if not self.randboss and self.rank >= 3.5 then
+			if not self.randboss and self.rank >= 3.5 and not self.no_difficulty_random_class then
 				local data = {}
 				if self.rank == 3.5 then data = {nb_classes=1}
 				elseif self.rank == 4 then data = {nb_classes=1}
@@ -483,7 +483,7 @@ function _M:addedToLevel(level, x, y)
 			for tid, lev in pairs(self.talents) do
 				self:learnTalent(tid, true, math.ceil(lev * 1.7))
 			end
-			if not self.randboss and self.rank >= 3.5 then
+			if not self.randboss and self.rank >= 3.5 and not self.no_difficulty_random_class then
 				local data = {}
 				if self.rank == 3.5 then data = {nb_classes=1}
 				elseif self.rank == 4 then data = {nb_classes=2}

game/modules/tome/data/zones/eidolon-plane/npcs.lua

@@ -30,6 +30,7 @@ newEntity{ define_as = "EIDOLON",
 	never_move = 1,
 	invulnerable = 1,
 	never_anger = 1,
+	no_difficulty_random_class = 1,
 
 	can_talk = "eidolon-plane",
 }