From Golden Sun Universe
Cloak allows Isaac to effectively walk invisibly along any shadowy area. The two guards at the gate to Lunpa Fortress are oblivious to the intrusion about to take place behind them.

Cloak (シャドー, Shadow) is a Mercury-class Utility Psynergy introduced in Golden Sun that costs 1 Psynergy Point. An Adept can obtain it if he or she is equipped with the Cloak Ball item, which you receive from Babi in Babi's Palace following the Colosso Finals.

The Cloak Psynergy causes the party to turn invisible while in a shadowy area. They will always materialize in light, however. It is used and required to complete the Lunpa Fortress sidequest, where it is used to sneak past patrolling guards in an instance of stealth gameplay. It is also used in Crossbone Isle, specifically the fourth floor, and that will allow you to progress through that floor's puzzle involving statues that block your path if they detect you. In that room, using both Cloak and Halt will also allow access to a chest containing the immensely useful Storm Gear.

The Cloak Ball and its Psynergy can appear in the inventory of Isaac's party in Golden Sun: The Lost Age if any form of data transfer is used. However, there isn't a single instance in the second game where the Cloak Psynergy is actually usable or useful.

Star mercury.gif Mercury 1 PP
"Hide away in shadows." ((GS)/(TLA)) Become invisible to certain NPCs and traps while remaining in shadowy areas.

Utility Psynergy in Golden Sun and The Lost Age
Golden Sun AvoidCarryCatchCloakDouseForceFrostGale/WhirlwindGrowthHaltLiftMind ReadMoveRetreatReveal
The Lost Age BlazeBurstCycloneGrindHoverLashParchPoundSandScoopTeleportTremor
Restorative Aura seriesCure PoisonCure seriesPly seriesReviveWish series
Mercury Psynergy in Golden Sun and The Lost Age
Elemental Base Damage Blue DragonCool seriesDouse seriesFrost seriesFrost CardFroth seriesIce seriesPrism series
Elemental Phys. Attack Call ZombieCutting Edge / Plume EdgeDiamond Dust / Diamond Berg
Support BreakCure PoisonElder Wood setPixie setPly seriesRestoreSuccubusWish series
Utility AvoidCloakDouseFrostLiftParch