RigaMortus
Explorer
Shin Okada said:I think you can declare "I move when the dragon come within 30ft from me." Then the dragon charges, come close to you, they you move. I think the charge fails. I am not sure if the dragon stops at there, or goes to the original destination, or can choose either of them. As a DM, I will say that the dragon foes to the original destination.
I'm not sure you can get so specific in a Ready Action. It's one thing to Ready against a charge (whether that charging NPC is coming from 40 feet away or 10 feet away), but I don't think you can specify and pinpoint exactly when the ready goes off. In the middle of battle, I think it would be very hard to measure off when a charging opponent is exactly 30 feet from you.