Boss 4: Soul Light

Setting
The players explore the crypt, and see the half-dead sister on an altar. Her soul is being drained, and the players need to restore her soul to awaken her. Thematically, the light of her soul is being dimmed, and has to be restored. The fight therefore revolves around taking action to combat the dimming, and taking action to restore light.

Overview
A crypt fiend is draining the soul of the sister. It periodically casts an AoE that damages everything in its line of sight. The crypt fiend has to be tanked in sight of the sister for her soul light to increase damage done, but has to be moved behind a pillar for the AoE.

Hardmode
The crypt fiend and the sister create beams of shadow and light, respectively, which empower a segment of the room. The shadow segment deals damage to players, and the light segment increases healing done.

Abilities

Diminish Soul: The crypt fiend periodically casts Diminish Soul, which damages everything in line of sight. This cast occurs when the crypt fiend's energy bar is full.

Rend Soul: When within proximity of the sister, the crypt fiend deals damage and splits off five fragments of the sister's soul every two seconds, which are interactables that can be sent back to the sister's body.

Soul Fragment: Soul Fragments persist for a short time, and are returned to the sister by interacting with them. If they are not interacted with in time, they dissipate. Returning 20 fragments removes all extra fragments, and spawns a Soul Orb above the sister.

Soul Orb: The soul orb can be healed, and empowers the sister's soul when fully healed. This increases the effect of Soul Light.

Soul Light: The sister periodically casts Soul Light, which increases damage taken by all enemies hit. As her soul is restored, this effect grows in potency

Soul Shadow: When the Diminish Soul cast finishes, the crypt fiend places a shadow zone on the ground. Casting Diminish Soul within a Soul Shadow causes the spell to ignore line of sight, and hit everything.

Hardmode Abilities

Beam of Light: The sister creates a beam of light, which covers an unshadowed segment of the room. Players standing within this segment deal increased healing.

Beam of Shadow: When the crypt fiend finishes a cast of Diminish Soul, the entire segment from the altar to the edge of the room is covered in shadow, which deals damage to any player standing within it.