Darkside is a mini-boss in Melody of Darkness. He, along with three others, must be fought to access Drachaos' chamber. There is no order in which to fight them.
Darkside only has two attacks, a staff swipe and a thunderbolt. The signal for the thunderbolt is when he points his staff to a cloud above him. The cloud will shoot a thunderbolt, which erupts into a shockwave when it touches the ground. He will teleport sometimes, when he appears, he will be a crystal until he fully materializes once more. You must hit the crystal to damage him. He has 1000 HP, and his theme is Resonance of Malevolent Souls.