When Shell is unleashed in battle, the user produces an effect that reduces all damage the entire party would take for the rest of the current turn by 60%. This effect has the "strikes first" property, meaning it will be the first performed action in a given turn regardless of the agility rating of the user and any other combatant in battle.
In Dark Dawn, Shell's unleash animation visually resembles the user summoning the 3D model of Shell above the party, which hovers around briefly as small light-blue energy orbs gather in front of each user in order, each briefly forming a glossy blue square shield-like shape that flashes in front of each Adept before disappearing.
Golden Sun: Dark Dawn: Shell is hidden in the small Gaia Falls Islet dungeon that is one of the dungeons you need to explore late in the game in order to collect the Umbra Gear. Enter the location and progress up until you have reached the chest that contains the Umbra Gear piece. Use the Grip Psynergy again to transport yourself back across the chasm below the chest. Now, step into the rushing river to your right through the small space in between rocks immediately to the upper right of this spire, walk right a little bit, and Grip the spire you see at the right end of your screen. Walk up from here and cast Grip on the spire at the left end of your screen, on the river itself. Run left from here, and you can access the Growth plant at the upper left corner of this area, which grants you access to the obscured doorway behind the waterfall. Collect Shell in the tiny room behind this waterfall without having to fight it. Now you can safely cast Retreat to leave the location.
General: Shell is one of several Djinn that cuts total damage the party takes this turn by a large amount, and does so with increased priority, which makes it a powerful defensive measure that can be relied on regardless of which Adept is unleashing it. Shell, a Djinni that lowers all damage taken down to two-fifths, is more useful than a Djinni that merely lowers all damage taken down to one half, like Bark. It would be well-worth using an Adept's turn to unleash this if Chasm, a Djinni that lowers damage down to one tenth, has not been found. In The Lost Age, the Mercury Djinni Shade is this Djinni's identical equivalent.
Golden Sun: Dark Dawn: Bizarrely enough, Shell is found very late in the game, and is found after the Venus Djinni Chasm, this game's strongest damage-cutting Djinni by far. Chasm is found earlier in Belinsk Ruins, and yet it cuts all damage the party takes by 90%, whereas Shell only cuts the damage the party takes by 60%. Unless you have permanently missed Chasm in Belinsk Ruins, this makes Shell far obsoleted as a Djinni to use; using Chasm makes the party practically damage-proof for a turn, while using Shell only cuts out a large portion of the damage the party would have taken. This means that Shell, at most, has only a few obscure benefits as a Djinni to consider for unleashing in battle; it would be Set onto a different Adept from Chasm, and it is possible to use Shell in between usages of Chasm. And this could only have some merits against the final bossand optional endgame bosses, which hit hard; random enemy encounters and most boss encounters in Dark Dawn aren't as threatening to the party as in the GBA games, making defense-oriented battle options in general, even one powerful such as Chasm, to be not as useful as offense-oriented battle approaches and group healing.