When utilizing Dark Shadow, Shadow does not change much physically. However, his fur and his eyes get a neon red coating and he gains an explosion-like neon red aura.
While in this form, Shadow becomes invincible and can use Chaos Blast, a destructive discharge wiping everything out within 20 yard range for a limited amount of time as well as use any weapon without draining its ammo/durability. In most cases, Dark Shadow can be obtained through committing dark actions and filling Shadow's Dark Gauge. The only other instance this form can be obtained is when fighting the Egg Dealer and causing Shadow Fever icons to appear on all three slots. This form can also be used in Shadow's Super State, which is Super Shadow.
Dark Shadow appears in the intro when he is being pelted with bullets by four G.U.N. soldiers in vain. He emerges from the fog and debris and uses the Chaos Blast to blow them all away.
It should be noted that Dark Shadow is a different ability compared to Chaos Boost; however, the two bestow Shadow with similar abilities, suggesting the two may be related.
Both Dark Shadow and Chaos Boost can only be used by Shadow.
In a hidden CG clip of the game, Shadow is seen approaching the camera as a silhouette from the thick cloud of fog. As his stripes start to glow red, an explosion occurs behind him and rushes at him.
When facing the Egg Dealer and initiating Shadow Fever, Dr. Eggman will announce his discontent at the slots landing on Shadow Fever, shouting, "N-no! Not Shadow Fever!"
All three slots landing on Shadow Fever on the Egg Dealer