Altar Emerald (アルターエメラルド,Arutā Emerarudo?) is the eighth Zone of Sonic Advance 3. The player immediately travels to it after defeating the Egg Gravity and clearing Chaos Angel. Once this Zone has been reached, it can be accessed again via Chaos Angel's Zone Map by moving just a bit further past the Boss Capsule and finding the Final Zone Ring.
Altar Emerald is a boss level; instead of a full-length platforming level, it is a simple straight path with two back-to-back boss fights (with Rings). Said bosses are G-merl and the Hyper Eggrobo.
After beating Dr. Eggman, if Sonic is the lead character and all seven Chaos Emeralds have been collected, G-merl will go rogue and harness the power of the Emeralds, thus the true final zone will appear, Nonaggression. If not, the normal ending will play in which the heroes attack Eggman, but the scientist escapes along with G-merl, followed by the Master Emerald restoring the planet to its original state.
The first boss of Altar Emerald is G-merl. During this battle, G-merl will attack the player with full-body charges, missile barrages in two different formations, or launch surprise attacks by teleporting into the middle of the field and then charge at the player. To damage G-merl, the player must attack him when he lets his guard down. After four hits he will be defeated.
In the Sonic the Hedgehog comic series and its spin-offs published by IDW Publishing, the Altar Emerald makes an appearance. In this media, much like in the games, the Altar Emerald was the site where Sonic once fought Gemerl.