Glad you're liking the new look, as for the end game I'm not sure how much I've posted here on it but the grand idea for the flow incorporates two major elements, the Chosen One and the Alliance AI.
The Chosen One is the hero foretold to defeat you, and depending on the archetype (sage, leader, adventurer) he will go about it in a different way, and if you want to win you should absolutely be paying attention to his strategy. A leader will rally the nations of the world together, mending bad blood and finding allies in strange places - you can expect the Alliance to have a strong military. A sage will quickly identify your nature, finding your weaknesses, and begin to spread tactics to dull your strengths - expect the Alliance to be well prepared to battle your agents and deal with your powers, and for the Chosen One to seek great artifacts and perform powerful rituals to seal you away. Finally the adventurer, the adventurer varies highly based on his class, but you are most likely to see the kind of cataclysmic final battle at the Old One's Seal with an adventurer type - he will find powerful heroes to be his allies, equip himself and his team well, and then prepare to smite you.
As for the Alliance and the way the world reacts to your powers, look at it this way. If the world begins to suffer from crazy earthquakes and waves of madness how does each nation act? A nation may turn inward, caring for itself and its own people, close its borders. Another may try to seize the opportunity to grab some neighboring land. These are good things for you, and how the individual AI can behave - the alternative is that they join the Alliance (the actual name is determined by the leader of the alliance), in which case they work together to solve what is plaguing the world. So yes, people may think something is wrong when the dead rise from their graves and stalk the land, but if you can make sure their response is to stay neutral until your victory is guaranteed that's the key to victory.
On a personal note, I ABSOLUTELY LOVE when the game comes down to final battles at the Old One's Seal. The number of agents and heroes that can be there is currently a "problem" for our GUI, but we're reworking combat right now to allow truly massive climactic battles.
So the other thing you mentioned was Karth - Karth wakes early as you said and he spawns Orc military units (called to his side) constantly while being able to perform other extremely powerful actions. He also can empower other agents with a similar power, meaning that you can raise a lot of expendable units. Yes, this may seem like you're exposing yourself to the heroes as an obvious Old One but whose to say what rumors reach civilized lands? Maybe its a god, or more likely just a 'powerful chieftain'... Managing the type of clues you leave, distracting the other nations, limiting WHO you attack and the SCALE of those attacks... the same mechanics apply even with a different pace.
Hopefully I'm not spoiling too much, part of the fun of the game is in understanding these mechanics as you fail and are sealed away again and again.