In this state, Eclipse looks similar to his normal form, but has increased in size, towering over Shadow. He has also developed four ridges on his forehead, in the shape of a crown.
The form seems more like an organic "suit" that forms around Eclipse's normal body, as seen when Shadow knocks Eclipse out of the form with a Chaos Blast.
Eclipse first used this form in an attempt to prevent Rouge and Omega from resetting their nuke in the New Black Comet. He engaged in heavy combat with Omega, enough to trump the robot, but he eventually reverted back to his normal form as his energy was drained.
In an attempt to kill Shadow and steal the Master Emerald, Eclipse used his Monster Form once more. He was able to overpower the hedgehog up until Shadow used the Master Emerald to enhance his Chaos powers and deliver fatal blows to knock the alien off of the altar.
Powers and abilities
In his Monster Form, Eclipse's strength and durability are greatly enhanced, enough to nearly exhaust Omega and easily defeat Shadow; by his own admission, Eclipse is ten times as strong as Shadow in this form.
This form severely drains Eclipse's energy. As such, he cannot maintain it for long, especially when it is used in heated combat.