The best Black Swan build in Honkai: Star Rail centres around buffing the fortune teller's DoT damage. This five-star character is laser-focused around dealing DoT—or damage-over-time—and supporting teams that also like to slowly chip away at enemies with wind shear, bleed, burn, or shock. 

Black Swan stacks the Arcana effect on enemies, dealing wind damage to them each turn, but she has a chance to inflict additional stacks on enemies who already have DoT effects. The more Arcana she stacks, the stronger her damage, plus she gets additional effects through her talent, such as ignoring 20% DoT defense when damaging enemies.

This means that if you really want Black Swan to shine, you'll whack her in a team that can apply the largest variety of DoT effects possible. Here's the best Honkai: Star Rail Black Swan build, plus all of her skills, eidolons, and ascension materials.

The best Black Swan build

Black Swan is a character focused around dealing DoT damage and gaining added benefits when placed in a team capable of applying lots of different DoT types. This is because the Arcana effect she inflicts gains extra stacks against enemies who already have DoT, buffing the wind damage she deals to them at the start of each turn, plus additional effects.

• Light Cone: Reforged Remembrance or Eyes of the Prey

Black Swan's five-star Reforged Remembrance Light Cone is arguably the best option for her. It boosts effect hit rate by 40%, which in turn buffs her damage once you unlock the Candleflame's Portent bonus ability. This Light Cone also adds a stackable resource called Prophet, which accumulates when the wearer deals damage to an enemy afflicted with DoT. Each stack increases attack by 5% and allows DoT damage to ignore 7.2% of a target's defence.

That said, you can only gain one stack per DoT that the enemy you damage has, meaning this Light Cone will only shine if you're using other party members to apply DoT effects also. This is the way you should use Black Swan since she inflicts extra Arcana, and so damage, when she hits enemies afflicted with a variety of DoT types. If you're using Black Swan in a team lacking the means to apply DoT, then Kafka's Patience is All You Need might be a better option if you already have it.

This Light Cone increases damage by 24% and speed by 4.8% when you attack, but more importantly, it inflicts Erode on targets, which makes them shocked at the start of their turn. This would be an effective way of allowing Black Swan to deal additional DoT and so stack more Arcana on enemies. If you're looking for a four-star Light Cone option, then Eyes of the Prey is your best bet for its effect hit rate and DoT boost. Though less ideal, you could also use Before the Tutorial Mission Starts if you grabbed it during the Silver Wolf event.

• Relics: Prisoner in Deep Confinement (Four-pieces equipped)

Black Swan's relic choice is pretty straightforward—four pieces of Prisoner in Deep Confinement for the 12% attack boost and the stacking 6% defense reduction on targets with DoT, up to three different types. If you're already using the Reforged Remembrance Light Cone, this effectively stacks the same effect for more defense reduction. 

For your primary stats, you'll want effect hit rate on the body to maximise the extra damage you're getting from the Candleflame's Portent bonus ability, plus speed on the boots is pretty standard for making sure Black Swan gets the chance to act often.

• Planar Ornaments: Pan-Cosmic Commercial Enterprise (Two-pieces equipped)

The Planar Ornaments for Black Swan are also simple, further bolstering effect hit rate with Pan-Cosmic Commercial Enterprise, plus adding an attack boost based on 25% of effect hit rate, up to 25%. In terms of primary stats, I recommend an attack boost on the Link Rope and wind damage boost on the Planar Sphere to maximise the devastating effect of those Arcana stacks.

Black Swan abilities and Eidolons

Here are Black Swan's abilities for leveling skills and the Eidolons you can get by rolling extra copies of her—as always, thanks to Honey Hunter for the details:

Black Swan Ascension materials

Here's what you'll need to ascend Black Swan, not including XP materials:

• 15 Extinguished Cores
• 15 Glimmering Cores
• 15 Squirming Cores
• 65 Ascendant Debris
• 300,000 credits (approx)

Extinguished Cores and their higher tier variants are easy to get through sending characters on assignment, or via defeating Fragmentum monsters in any other activities. Ascendant Debris comes from the Stagnant Shadow: Shape of Celestial in the Alchemy Commission area of the Xianzhou Luofu. Lastly, you can get credits by doing just about anything, but if you need them fast, the Golden Calyx: Bud of Treasures is your best bet.