The Four Eye Stones
The various colored Eye Stones are used for multi-player situations. They are activated from your "consumable" inventory (although they are never consumed), and you may equip them in your quick use  slots to activate there. Do not confuse them with Stones of Ephemeral Eyes which are different, as they revive you to your body form and they are consumed when used.
This stone is obtained from the Maiden In Black after defeating Phalanx and speaking with the Monumental on the third floor of the Nexus. It allows you to leave your Blue Soul Sign when in soul form, letting other players summon you to their worlds in the form of a Blue Phantom.
This stone is obtained from the Maiden In Black after defeating Phalanx and speaking with the Monumental on the third floor of the Nexus. It has two purposes. When a character is in body form, it allows the host to remove Blue Phantoms that had been summoned and send them back to their own worlds, but unlike the Resurrection spell, it does not revive Blue Phantoms back to their body forms.
When a character is in soul form, it allows the guest (or invader) to leave a world to which he's been summoned (or has invaded). Blue Phantoms can use the White Eye Stone without being penalized. However, if a Black Phantom uses a White Eye Stone to leave the host's world, after having invaded using a Black Eye Stone, that Black Phantom character will lose a Soul Level upon returning to his world.
When a Blue Phantom assists in a world but is sent away via the White Eye Stone before the boss fight and is sent away "With Thanks," the Blue Phantom receives a significant bonus (double) in Souls accumulated during the co-op session.
This stone is obtained by killing an enemy Black Phantom (when offline, the first NPC Black Phantom killed will drop it). It allows you to invade another player's game world as a Black Phantom. If you are able to kill the other player, you will regain your body in your own world. If you get killed by the host or a Blue Phantom, there is no penalty, but if you die because of environmental hazards (such as falling off of a cliff), you'll lose a Soul Level upon returning to your world.
This stone is obtained by killing the Maiden In Black instead of allowing her to put the Old One back to sleep at the end of the game. It allows you to put a Red Soul Sign on the ground, letting another player summon you into their world as a Black Phantom for a player-versus-player fight. The loser will lose a soul level, the winner will gain souls equal to that lost Soul Level's cost.
Using Eye Stones
Note there are some regions in each level where the game does not allow using colored eye stones. Try equipping a colored stone in your  use slot, walk around and observe it change from ghosted near an archstone (stone not usable) to un-ghosted (usable) as you move around. Look for messages like "You can summon here", often near the start of a level and just before the boss door, for the most common places to use the stone to ask for/offer help. Also, each level is separated into zones divided by the fog door locations you encountered the first playthrough. A stone sign you place is only good while you are in the same zone of the level, so when you pass the location of a fog door, any stone sign you have left in the previous zone will "fade", and you can place another in the current zone as you play on. You will experience this if you put down a Blue Soul Sign, but instead of waiting around for a summons, just proceed through the level yourself while waiting.
|Soul||Blue Eye Stone||A gift from the Maiden In Black - after defeating first boss||offer yourself to be summoned and help in co-op mode|
|Body||White Eye Stone||A gift from the Maiden In Black - after defeating first boss||dismiss Blue Phantoms you are co-oping with|
|Soul||White Eye Stone||A gift from the Maiden In Black - after defeating first boss||leave another host player's world you are in, if you are a Blue or Black Phantom|
|Soul||Black Eye Stone||Drop from the first NPC Black Phantom you defeat||invade a random host for PvP|
|Soul or Body||Red Eye Stone||Killing the Maiden In Black in the End Game||offer yourself to be summoned for PvP duel with host as a black phantom, when used in body form, you will be transported to the duel in soul form, with the known lower HP values of course, in case you die, you will return to body form, regardless.|
|Soul||Stone of Ephemeral Eyes||- Treasure - throughout game
- Enemy drop - throughout game
- 5 -1/2 - Sold by Filthy Woman
|revive yourself to body form at any time, with full HP and MP only in your own world (not as blue or black phantom)|