Moonlight Butterfly

The Moonlight Butterfly is an enemy in Dark Souls encountered as a boss in Darkroot Garden and as a regular enemy in the Crystal Cave.

Moonlight Butterflies are one of Seath's many creations. A very large butterfly, it is one of the most mysterious creatures in Dark Souls. Aside from its strong magical powers, it has no visible head or legs and shows a wheel-like device between its upper wings. Moonlight Butterflies are capable of firing various magic attacks, but they don't have any melee capabilities.

The Butterfly in the Darkroot Forest is a boss and attacks as soon as its room is entered, but those found in the Crystal Cave are passive and will not attack the player unless provoked.

Darkroot Garden

 * It is possible to summon Witch Beatrice in the destructible shrubs underneath the stairs, just before the boss (as long as the player has reversed hollowing). Her magic attacks, despite some accuracy issues, do colossal damage to the boss, making it possible for Beatrice to kill the butterfly by herself before the player even has a chance to attack it themselves.
 * The Moonlight Butterfly's attacks are magic based. Wear light armor with high magic resistance (physical resistance is irrelevant), so you can dodge quickly and mitigate damage in case you get hit.
 * The butterfly spends most of the fight outside of melee range. While it's possible to defeat it using only melee weapons, players lacking ranged attacks (i.e., a bow or magic) are left to dodge (or block and heal) until it lands. A ranged weapon is therefore strongly advised, but not one that locks the player into long charge / reload animations (i.e., don't try to use a Heavy Crossbow), which would make it much harder to dodge the projectiles. The priority during the flight phases is to stay alive (and, ideally, take no damage at all).
 * During the flight phases, try to keep as far as possible (though within targeting range, naturally) to make it easier to identify and dodge the butterfly's attacks.
 * All the attacks can be dodged, resulting in zero damage taken. As a general rule, dodge towards the projectiles (and through the laser beam) just as they're about to hit you, but away from the blue orb (Soul Sphere), before it gets too close to you.
 * When the butterfly is on the fog gate's side of the bridge, you can use the gate itself to block its attacks.
 * When the butterfly lands, switch to a melee weapon (preferably two-hand) and hit it for as much damage as possible (it won't fight back during this phase). Equipping the Grass Crest Shield will improve stamina regeneration and allow the player to get more hits in. For magic users, switch to a high-DPS spell such as Combustion or its improved versions.
 * The Butterfly gradually restores lost Health when it lands, and although it isn't a significant amount, one should still make sure to deal as much damage as possible to avoid extending the battle.
 * If you prefer to stay at range with a bow when the butterfly lands, keep in mind that locking onto the butterfly's center will probably cause projectiles to hit the wall without causing damage. One can continue to shoot the butterfly's wings manually by disengaging the lock-on, and since the butterfly's only attack in this phase is close-range, the player can keep firing with impunity until it takes off again.
 * A few seconds after landing, it will glow and emit noise, just before releasing a large area-of-effect attack. Move out of range before the attack goes off to avoid significant damage. Switch back to your ranged weapons / spells, and repeat until the boss is dead.

Crystal Cave

 * The Butterflies in the Crystal Cave are not aggressive, so the best strategy is to leave them be. If attacked or approached too close to, they will never land to rest, therefore some form of ranged attack is absolutely necessary.
 * The laser attack deals significant knockback, and the platforms in Crystal Cave are very small; care must be taken to dodge precisely.
 * It is possible to use a bow to take down the butterflies from afar without making them attack the player. The Hawk Ring is recommended for maximum damage.

Trivia

 * The Moonlight Butterfly shares the same theme with Dark Sun Gwyndolin.
 * The Moonlight Butterfly's name could be a reference to the Moonlight Butterfly from Turn-A Mobile Suit Gundam.