Petra

Location
Petra is a Djinni that is randomly encountered in a specific area of the overworld that otherwise appears to be pointless scenery. Thus, it is very often missed by players playing the game the first time without any outside information. The battle with Petra may ensue in place of a random battle in the pictured area in northeast Hesperia in a vertical "V"-shaped stretch of forest; this region is most easily reached by sailing the ship into the river opening on the northeastern tip of the landmass. Petra then must be battled to be acquired.

As an opponent
Statistically, Petra has 830 HP, 77 PP, 260 Attack, 79 Defense, 227 Agility, and 26 Luck. Like all other Venus Djinn that are fought, in terms of resistance, it has a Venus Resistance rating of 193, Mercury and Mars Resistance ratings of 100 each, and a Jupiter Resistance rating of 25, and in regards to its abilities it uses its available Venus-based attacks with a Venus power rating of 125.

Petra can use the following battle commands:
 * Stone Spire: Used 53 out of 256 times, this is a Psynergy spell that causes five large stalactite to fall down from above onto the party and shatter, dealing a Venus-based attack with a power rating of 160 and a range of 3. This ability consumes 22 of its user's PP.
 * Briar: Used 47 out of 256 times, this is a Psynergy Spell that generates a mass of thorny spiked vines at the position of the party of Adepts, dealing a Venus-based attack with a power rating of 70 and a range of 3. It costs the user 11 PP.
 * Wild Growth: Used 41 out of 256 times, this is a Psynergy spell that causes a large amount of thorny vines to erupt from the ground and though the party, dealing a Venus-based attack with a power rating of 110 and a range of 5. It consumes 19 of the user's PP.
 * Mother Gaia: Used 35 out of 256 times, this is a Psynergy spell that causes the ground beneath the enemy to open up and engulf the party in a wider reverse shower of Venus energy and medium-sized boulders, dealing a Venus-based attack with a power rating of 100 and a range of 5. This ability consumes 17 of its user's PP.
 * Quake Sphere: Used 29 out of 256 yimes, this is a Psynergy spell that prompts three rectangular sections of ground underneath the party to jolt upward and retract into the ground quickly thrice in succession, bouncing each Adept three times, dealing a Veus-based attack with a power rating of 65 and a range of 7. This ability consumes 15 of its user's PP.
 * Attack: Used 28 out of 256 times, this is this monster's standard physical attack, but for this monster it is counted as a Venus-based attack rather than a non-elemental attack.
 * Flee: Used 23 out of 256 times, this enemy has the ability to attempt to Flee the battle, thereby ending the battle if successful without rewarding you with the associated EXP, Coins, and the Djinni itself.

When defeated, Petra yields 937 EXP and 631 Coins, and the Petra Djinni is added to your party's Djinn collection. If you fell it with an offensive Jupiter Djinni like Wheeze, its rewards increase to 1218 EXP and 820 Coins.

Very easy to destroy in one turn with summons like Thor, which you really should have considering this is found in the Great Western Sea segment of the overworld much later in the game.

Description
When Set, it increases base HP by 11 and base Defense by 3.

When Petra's battle effect is used, a holographic orange image of a Venus Djinni projects a stream of orange, gaseous energy at the target, causing the target to be "unable to move" for one of its actions this turn (meaning the enemy cannot attack at all this turn if it can only act once per turn, whereas a boss that can act twice a turn can only act once a turn). This acts first in battle regardless of the Agility ratings of the user and the target, and it shares its priority with other "acts first" moves like the opponent's Shade and Flash Djinn in VS mode (meaning that the Adept with the higher Agility will use his acts-first move first). It is identical to the Golden Sun Djinni Ground.

Ability analysis
This is different from the Stun status condition in several important ways: It always works, and it has the top priority of any move in a given turn so it will always hit first, but it will only negate one of an enemy's actions instead of preventing it from acting at all for potentially several entire turns. Depending on your class setup, this is a great command for a supporting Adept to use to partially prevent a boss from damaging the party (it probably isn't a good idea to have Felix use it when he should be focusing on dealing damage, though). Late in the game when Isaac's party joins Felix's with all of the previous game's returning Djinn in tow, the Venus Djinni Ground becomes Petra's identical and equally useable counterpart.

One particularly great use of Petra or Ground is in the third and final stage of the final boss fight against Doom Dragon; the two moves Doom Dragon is allowed to use will almost always both be extremely powerful and deadly, so preventing one of them will be a huge help in surviving the end of the battle. Another especially under-appreciated use of this effect manifests in the start of the Star Magician battle; Petra and Ground sharing the same priority as the Guard Aura monster skill the Guardian Ball uses to make the Star Magician invulnerable with means that if the user of Ground/Petra has higher Agility than the Guardian Ball (though the Guardian Ball has one of the highest agility ratings in the game, with 292 agility), the user can outspeed the Guardian Ball and make it unable to use Guard Aura with Ground/Petra, thus ensuring that the other three Adepts this turn can fully and safely make use of the strongest summons all together. This Djinni can singlehandedly make a very tricky and challenging optional boss battle much more simple than otherwise possible, in other words.

When you have both Djinn in your collection, the two together can completely prevent any enemy that only acts once per turn from ever attacking. While Adept One uses one of the two Djinn to freeze the enemy, Adept Two re-Sets the other of the two Djinn that was used to freeze the enemy in the previous turn, and the next turn Adept Two does the freezing while Adept One resets his Djinni. Adepts Three and Four, in the meantime, are the fighters who safely deal damage. This alternating strategy can be infinitely performed to ensure complete safety for the Adepts throughout the battle (though of course the battle would be a lot longer and more tedious this way). This is not as insanely helpful against any boss that can act more than one turn, though, since its second attack will not be prevented - and the reason this strategy never caught on is that none of the bosses following the reunion act only once per turn to begin with.

Name Origin
Petra is an abbreviation of the word petrified, which refers to any organic material that is changed to stone (hence the reason the Djinni Petra causes targets to become immobile). The most common case of this is petrified wood.

Interestingly, Petra is also the Greek word for rock. There is yet another Petra, this one an ancient city carved out of the cliffs in Jordan.