You are probably wanting to do Herald hunt I assume? Might want to think about this, and here is why.
First you terrorize the Act, next you need to start from the very beginning of the act and kill everything. Next hope for a Herald to spawn. Worst case scenario, you can actually get the "A Herald is hunting you" when you have basically cleared the area. So it never spawns. Continue on killing, after a few hours or so (IN THE SAME #*$& ACT) you may have 2 to 3 Heralds killed. But that is nowhere close to what you need, so use another shard and terrorize another act. Hopefully after another couple hours you will have killed 4-5 Heralds. Now you can hope the RNG gods do not want to f**k you.
So you just Terrorized 2 acts, got 5 Heralds, nothing so far. Spent about 4-5 hours, but cant stop there. Now Terrorize another act, and start hoping for Heralds. Spend 8 hours, 10-15 Herald kills, and prepare to basically come away with nothing. Countless people have spent 8-10 hours, terrorizing up to the entire game and walk away with nothing. This is a stupid game metric mechanic to try to get players to spend more time in game so they can use this for marketing crap. You would be better off just farming for 8 hours, probably find more stuff, and in the end have had fun.