For the next 3 seconds, Xin Zhao is invulnerable against enemy champions more than 450-units away from him. Basic attacks and other spell casts extend the duration by 0.3 seconds, but the remaining duration cannot exceed 3 seconds.
Pets, clones, traps, bombs, DoTs, and delayed-damage skills will not damage you if the enemy champion source is outside the circle when the damage applies.
Enemy minions, towers, and jungle monsters will damage you even when they are outside Crescent Guard's range.
Crescent Guard does not negate crowd control effects, it only negates the damage portion.