Why do I come to the plain and I can't see Groth的
According to Orc's War Event logs, first, you need to talk with Orgram to start the event.
After that, you need to go to Plains of Death. Please make sure you be there during daytime (6.00 - 19.59).
Then, select "Path to the shine." It should automatically trigger the event which lead you to fight the red orc.