The Ancient Light is an upgrade for Sonic and Shadow in Sonic Adventure 2. Obtaining the Ancient Light allows both Sonic and Shadow to perform the Light Speed Attack, an attack that destroys multiple enemies at once.
Sonic can find his Ancient Light in Green Forest, which serves as an optional upgrade. After the second time Sonic needs to swing on vines and after coming through the second loop (half loop), he has to hit the springs and land on the desired platform. Once on this platform, go to the right and there will be a 10 rings Item Box. Go to the nearest edge and a Mono Beetle should appear, and use a series of Homing Attacks on it and the Item Boxes that appear behind it to reach another platform, where the the Ancient Light are located.
Shadow can find his Ancient Light in Sky Rail, which serves as an optional upgrade. Left from the first Chao Container, which is located on the platform to the left, before taking the first rocket up, is a line of Mono Beetles. Use the Homing Attack on them until Shadow reaches the spring on the other side of the pillar, which leads up to the Ancient Light on top of it.