Ancient Light

The Ancient Light is an upgrade that can be obtained in Sonic Adventure and Sonic Adventure: Director's Cut by Sonic and by both Sonic and Shadow in Sonic Adventure 2 and Sonic Adventure 2: Battle. Obtaining the Ancient Light allows both Sonic and Shadow to perform the Light Speed Attack, an attack that destroys multiple enemies at once.

Sonic Adventure
In Sonic Adventure, the Ancient Light is located in the Mystic Ruins Adventure Field when Sonic has to proceed to Red Mountain. Go into the cave that appears in front of the train station in the Mystic Ruins and go through the tunnel, until Sonic gets to the other side, with Angel Island to the left. Pass through the field, until Sonic gets to the entrance to the bridge leading to Angel Island, and the Ancient Light will be located on a flat rock platform to the left of the bridge entrance. With it, Sonic is able to proceed onto Red Mountain Action Stage, by destroying the Baby Kiki guarding it.

Sonic the Hedgehog
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 be 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 the Hedgehog
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.