Tethrys

Tethrys is a Rift Guardian, only encountered in Nephalem Rifts as a random boss. She is a greatly empowered Succubus.

In addition to increased damage and Life, Tethrys also gains the following abilities:
 * Fireball Volley: periodically releases 3 relatively small projectiles dealing high Fire damage on impact.
 * Blood Star Volley: unlike normal Succubi, Tethrys summons them rarely, but 6 at a time, and they appear either behind her or within 20 yards of a player, drifting towards the target. These Blood Stars promptly apply the curse, reducing Armor. Unlike normal Blood Stars, these are actually very fast.
 * Teleporter affix (with reduced cooldown).
 * Knockback affix: any projectile attack has a chance to cause Knockback on successful hits.
 * Doomfire: creates 6-12 pools of dark matter on the ground, roughly 10 yards in radius each, anywhere within the boss's line of sight. Each of them fully forms after 3 seconds, and exists for 2 seconds, dealing very high Physical and Fire damage over time if players stand in those. Has a 30 seconds cooldown, only used when Tethrys is below 50% of maximum Life.
 * Claw Swipe: only used if all other attacks are on cooldown; hits the player in melee for average Physical damage.

This boss is most dangerous to melee characters, as her abilities do not have minimum range, and Armor reduction makes the total damage received insanely high.

Upon death, she had a chance to drop Dark Bat Wings in patch 2.4.1 PTR, but does not do so in live version.