The Grand Helm is the ultimate heavy armor helm in Final Fantasy XII that boosts the wearer's magickal defense by 50 (Zodiac) or 40 (original), Strength by 12 and—in the Zodiac versions only—Vitality by 10, the latter making it good for a Foebreaker who wields axes, hammers and hand-bombs that take the wielder's Vitality into the damage calculation. The Grand Helm is metallic for the purposes of the magnetize debuff present during the Vinuskar (though the helm is available after this boss) and Ultima boss battles.
The Grand Helm is notorious for how difficult it is to procure in the original version, where it also only has marginally higher stats than the preceding Magepower Shishak, making it more of a completionist's trophy than a coveted endgame equipment piece. In the Zodiac versions, it now also boosts Vitality and there are more ways to obtain it, making it a good contender for an endgame helm.
The easiest way to obtain it is to start in Mosphoran Highwaste and get the maximum chain level with Clay Golems, east of the Babbling Vale. The player should not pick up the loot to achieve the maximum chain level faster. At the maximum chain, the player must not enter the Babbling Vale, or the chain will be lost. It is best to turn off attack gambits as not to accidentally break the chain, and take the nearby chocobo by giving it Gysahl Greens, and head south to the Nalbina Fortress and then to the Barheim Passage.
The player can board the Strahl and fly to the Ridorana Cataract. The player must not touch any save crystals or kill any enemies, or the chain will break. At the Pharos the player must make their way to to the Dais of Ascendance 88F, where Tower spawns 20% of the time. If Tower does not spawn, the player can exit and reenter the area by touching the waystone. If the player has not killed any enemies or touched any save crystals they will continue the chain. When killing the Tower, it is important to try and prevent it from falling off the ledge when it dies, or it will not drop loot.
If Tower does not drop the Grand Helm, one can go to the first floor and move to the 3rd Ascent - Spire Ravel - 1st Flight again, or go and touch the colored sigils in the correct order: (black, green, red, sacrifice, and the clear sigil hidden offscreen). Tower will continue spawning and the player can obtain multiple Grand Helms if they are patient.
In the updated Zodiac versions, the Grand Helm is still the rare very drop from the Tower, and the same chaining trick works, but there are now less arduous ways to procure it. The Grand Helm is found as a treasure in Henne Mines' Phase 2 Dig (25% chance to appear, but is always the helm), and Cerobi Steppe's Feddik River (10% chance to appear, 80% chance for gil, 5% chance the item treasure is the Grand Helm with the Diamond Armlet equipped).
Grand Helm is the ultimate helm giving 50 or 40 Magick Resist depending on version and 12 Strength, as well as 10 Vitality in the Zodiac versions. The latter is useful for a Foebreaker whose native weapon types account for Vitality in damage calculation. Vitality also affects status resistance and duration, and thus is a good boost to have on any job. However, the Magepower Shishak, which offers +5 to Magick Power, could be the ultimate helm for a katana-user, although in the Zodiac versions, Bushi can only use the shishak with Zodiark unlocked.
The strength bonuses make heavy armor ideal for frontline fighters, and the Grand Helm is a good helm to equip on anyone who attacks physically, although a gun-wielder does not benefit from the strength-boost. In the Zodiac versions, Uhlan, Knight, Time Battlemage, and Foebreaker can equip it natively, and Archer can unlock it with Shemhazai. Grand Helm is only useful to obtain if the player has chosen these License Boards for the party. In The Zodiac Age versions, after fighting Belias the player can choose two License Boards per party member and so more characters can benefit from heavy armor, as it is usually a good idea to match jobs that use different types of armor to bolster light armor jobs' strength, and to give more durability to spellcasters.