Dark Soul

A Dark Soul is a NetNavi's soul which is triggered and engulfed by a Dark Chip's influence. DS Navis first appeared in MegaMan Battle Network 4 with the introduction of Dark Chips. DS Navis, especially who created specify for combats, will most likely cannot become normal after the Dark Chips's effects wear off. The only way to get out of a Dark Soul's influence is wait for it to separate from the NetNavi, except on rare occasions, the NetNavi gains control of itself. It is possible to turn any Navi into a DS Navi, and evolve any Dark Soul into a Darkloid, which are much more powerful than DS Navis.

Abilities
Generally, the Navi's body turns blackish purple and itself gains extra abilities. Gameplay-wise, the operator gains access to NetNavi's Dark Soul any time they use a Dark Chip in battle. While under the effects of Dark Soul, the NetNavi can use Battle Chips exclusive only to Dark Soul, like Evil Chips. Additionally, once per battle, if he would be deleted, he instead remains at 1 HP, turns completely black, and becomes invincible but uncontrollable, moving and attacking randomly with Buster Shots and previously used chips and Program Advances. While NetNavi is "berserk", the player is still able to open the Custom Window and use loaded chips at will.

MegaMan in particular, is able to use DS Navi chips in DS mode, but in exchange for being able to wield the darkness, loses the ability to perform Double Soul, cannot enter Full Synchro, and cannot use certain Battle Chips like Bug Fix, Sanctuary, or SP Navi chips neither. The effects of MegaMan's Dark Soul can be remedied by battling without using Dark Chips, a process that can be sped up using the Soul Cleanser Navi Customizer part. In both Battle Network 4 and 5, all NetNavis that the player can battle in the game have a DS version of them, even Bass (Bass XX). A DS Navis' HP and attack strength are determined by MegaMan's DarkSoul Level, and is 1% of MegaMan's base HP (or the first number). The number isn't rounded.

MegaMan Battle Network 4
MegaMan uses a dark chip as the only way to defeat ShadeMan. Throughout the game, MegaMan is faced with the temptation of using the power of Dark Chips, but is able to suppress his dark soul enough to keep under control. Later in the game, it is discovered that Chaud used a Dark Chip on ProtoMan as a last ditch effort to delete 500 evil Navis. ProtoMan's Dark Soul however, had taken over him over and he became evil, detaching himself from Chaud. MegaMan chases ProtoMan into the Undernet and confronts him. Under the operation of Chaud, MegaMan is able to defeat and capture ProtoMan DS and his dark soul is suppressed. At the game's climax, MegaMan is forced to confront his separated Dark Soul in battle before fighting Duo. MegaMan's Dark Soul's fighting style is very much developed based on MegaMan's and has the same HP and Navi Customizer parts equipped. MegaMan defeats his Dark Soul and then continues on to defeat Duo.

MegaMan Battle Network 5
MegaMan gets kidnapped by Nebula forces and receives a "Dark Chip implant" turning him into Dark MegaMan. Nebula places him in charge of controlling End Area 2 and only returns to normal after being defeated by the liberation team. Also, in Nebula Area 6, DS versions of friendly Navis in the game exist independently from the original NetNavi's body. Dark Soul was deleted after MegaMan became Hub and defeated Nebula Grey.

MegaMan NT Warrior
In the anime, Navis using Dark Chips will start to develop blotches of black or purple on its body. It should be noted that this is only shown happening to FlashMan and DesertMan. Also, MegaMan turns dark after a painful bite to the neck from ShadeMan.

Trivia

 * Three Dark Souls evolved into Darkloids and exist on their own, they are: Dark MegaMan.EXE, Dark ProtoMan.EXE and Dark Colonel.EXE.
 * While some DS are separated from the original NetNavi's program, it is still possible for those NetNavi to enter DS mode by using any Dark Chip. This may lead to a conclusion that the possible number of DS that can be created is infinite.