Phantom

Phantoms are a gameplay mechanic in Dark Souls. There are two main varieties of phantoms which enter the players world: Summons and Invaders.

White Phantoms
White Phantoms are either other players, or are specific NPCs, which are summoned to the player's world via a White Sign Soapstone. As a result, they can help the summoner defeat enemies and bosses, until either the area's boss is defeated, the summoned or summoning player dies, or if the summoning player uses the Black Separation Crystal. No rewards, apart from souls and one humanity, are granted to white phantoms.

Gold Phantoms
Gold Phantoms are distinguishable from other summons by their golden color, which is a result of them being in the Warrior of Sunlight covenant. The purpose of a golden phantom is the same as a white phantom's, however, upon successfully helping the summoning player to defeat the area's boss, they will be rewarded with a Sunlight Medal, in addition to souls.

Both White and Gold phantoms receive half the souls an enemy or boss would drop upon death in the summoner's world. This means that, if the summoned player is in NG and the summoner is in NG+, the former will often end up getting more souls for a kill as a white phantom than he would in his own world (ex., if a boss is worth 40k souls in NG and 120k in NG+, then an NG player summoned to help with the fight in NG+ will get 60k).

There is no change to the rewards received by the summoner.

Red Phantoms
Red Phantoms, otherwise known as Black Phantoms or Dark Spirits are the basic type of invading character or player. Their red coloring is caused by the invader either being in the Darkwraith covenant, using a Cracked Red Eye Orb, or being summoned to another player's world through placing a summon sign with a Red Sign Soapstone.

The aim of a Red Phantom is to kill the host whose world they have invaded. Upon the host's death, they are rewarded with liquid humanity.

Blue Phantoms
Blue Phantoms are unique in that they only invade specific people. Blue Phantoms are members of the Forest Hunter or Blade of the Darkmoon covenant, or victims of a member of the Gravelord Servant covenant, and thus only invade those who are deemed "guilty". Their purpose is to punish the appropriate player by destroying them. If they are successful, they will be rewarded with a covenant item upon the host's defeat.

The player is never invaded by an NPC Blue Phantom in the game, however, they can invade Lautrec the Embraced as part of a side-quest.

Dragon Spirits
Dragon Spirits are members of the Path of the Dragon covenant, and invade other players in order to obtain Dragon Scales. Their most unique feature is that the player's character will not change color like other phantoms do. In this regard, Dragon Spirits can be considered to be 'true' Black Phantoms, as their armor and weapons simply take on darker shades of their natural colors.

List of NPC Phantoms
* Can only be found in the Artorias of the Abyss DLC.