Lull

Location
Lull is present on one of the stone rooftops in Loho. It can only be accessible very late in The Lost Age because you need the Magma Ball collected at the end of the Magma Rock dungeon, which is itself only accessible after Isaac's party joins Felix's late in the game. When you have the Magma Ball, simply use it while standing behind the pointed cannon at the east end of town; after the wall it is pointed toward is removed and a cutscene transpires where the cannon is installed onto your ship, simply climb up the vine and go to the lower left, and hop onto where Lull is to collect it.

Description
When Set, it increases base HP by 11 and base PP by 6.

When Lull's battle effect is used, a holographic effect of a Jupiter Djinni floats above the battlefield and projects an elaborate, swirling ring-shaped array of musical note images. The turn immediately ends; while all battle commands executed prior to Lull's usage this turn are unaffected, all of the battle commands that would have followed Lull's usage otherwise simply do not happen.

Ability analysis
Lull's unique ability comes across as overly complex to be useful for portions of the player-base because it takes a lot of thought and party organization to make use of Lull's effect meaningfully. But such usages exist; easily the greatest use of Lull is for all four currently battling party members to have a higher agility rating than whatever boss or big enemy is being fought, and for the Adept among them who has Lull Set onto him/her to have the lowest of these agility ratings. After the other three party members attack the boss quickly and safely, Lull is used so that the boss will be prevented from doing absolutely anything this turn. This can work wonders on the optional boss Dullahan in particular; as this YouTube video demonstrates, properly timed usage of Lull allows for the most powerful and deadly optional boss of the series to be able to be defeated in two turns without the boss being able to execute a single battle command.

Name Origin
A lull is a short rest, most likely why this Djinni causes a cease-fire.