Dark Shiva
Dark Shiva is one of the Dark Aeons and appears once you return to the road outside Macalania Temple at any point after defeating Seymour Natus in Bevelle. You do not need the airship to trigger her, so be careful if you walk back to the temple early to pick up a missed Destruction Sphere treasure. Dark Shiva blocks the pathway that you would take to re-enter the Macalania Temple. There is no way to get inside the Temple until after you have defeated her. Her blocking the path means that if you did not complete the 
Destruction Sphere portion of Macalania Temple, you will not be able to obtain Anima until after you defeat Dark Shiva.
Note that the Dark Aeons only appear in the International, PAL, and HD Remaster versions of Final Fantasy X. They are not present in the original North American PlayStation 2 release.
Location
The Dark Aeon is located just outside the Lake Macalania Temple. Warp to Lake Macalania and take the pathway back towards the Temple. As you approach, one of the guards will bar your entry and will attack you with Dark Shiva.
Battle Preparation
Each of the Dark Aeons, including Dark Shiva, is considered a superboss, and they are some of the strongest enemies that you can fight in the game. As such, you will need to ensure that your party is fully leveled, has their best weapons, and has their stats maximized as much as possible. Refer to the Advanced area of the Tips and Tricks section for additional details.
Tips and Tricks
Customizing strong weapons and armor, quickly leveling your characters, maximizing their stats, and other helpful endgame tips and tricks.
Equip your characters with armor that has the 
Auto-Haste ability, as Shiva is very quick, and she will get in a ton of attacks if your characters can’t match her speed. You will also want to make sure you have the Auto-Phoenix ability on your character’s armor.
Dark Shiva Stats
HP: 1,100,000
Overkill: 99,999
AP: 20,000 (30,000 for an Overkill)
Elemental Affinities: Absorbs ice damage, halves fire damage, immune to lightning and water damage
Immune to Sensor
Battle Strategy
Dark Shiva is not too difficult to defeat as long as you can land attacks on her. Use 
Quick Hit often as it will ensure you can get in as many attacks on Shiva as possible. Ideally, you want to defeat her before she gets an opportunity to use her Diamond Dust Overdrive ability.
Similar to the battle against the other Dark Aeons, you need to be prepared to deal with her Overdrive attack, which will instantly KO your party members. You can either have Yuna summon an Aeon just before the attack or have your party members cast 
Auto-Life on one another. Just be careful, as some of Shiva’s attacks will remove the Auto-Life effect.
An onslaught of guards will attack your party after the battle, just like the last time you escaped from Macalania Temple. Continue running until you reach the Crevasse area to stop the Guado Guards from pursuing you.
Items and Rewards
You can Steal the following items during the battle:
Mana Tablet (x2) (common)
Elixir (rare)
And Dark Shiva will drop the following items when it is defeated:
Dark Matter (common)
Master Sphere (rare)
Dark Shiva will also always drop a weapon or piece of armor with three or four ability slots. Her weapons can include abilities such as Icestrike and Break Damage Limit, while her armor can include Ice Eater, Break HP Limit, and Ribbon.
Helpful Hint
In the HD Remaster, the Guado may still be blocking the entrance to Macalania Temple immediately after you defeat Dark Shiva. If this happens to you, exit the area and then return, and the entrance should be clear. Some players have also reported that the Guado will not leave for good until they have visited Bevelle a second time.