Sethria is a twilight dragon found on Sethria's Roost in Mount Hyjal.
Using Shadow Cloak Generators she has hidden
Aviana's egg from the druids that would reclaim it, nestled amongst her eggs at the Roost. It seems that Sethria is behind the harpy situation at the Shrine of Aviana, as revealed by Marion Wormwing when she is captured and interrogated for the quest .  A Bird in Hand Thisalee Crow and the Druids of the Talon help players in defeating her.
Human form : She has ~371,000 health and the following abilities:
40 yd range — Inflicts Shadow damage. Twilight Bolt 2.0 sec cast. 1400-2000 Shadow damage
50,000 yd range — Deals 11795 to 15102 Shadow damage to any enemy standing within the Shadow fissure after 5 sec. Twilight Fissure 1.6 sec cast (5 sec cooldown) 50,000 yd range — Deals Shadow damage to any enemy standing within the Shadow fissure after 5 sec. Void Blast 2400-2700 Shadow damage despite the tooltip
Dragon form : She has 557,000 max health/40810 max mana in her dragon form, but she is at the same ratio of health/mana she was at before the shift (now roughly 400,000 health).
Melee swings for 600-1000 Physical on cloth
15 yd range — Inflicts 5063 to 6171 Twilight Breath Shadowflame damage to enemies in a cone in front of the caster. Instant. Hitting for 11-1700 Shadowflame, despite the tooltip.
Choluna says: Mother of all winged creatures hear my prayer and reveal yourself...a vision! what could this mean, <player>? I will report this to Omnuron.
Vision of Sethria says: This is the one. The ancient will be ours. Vision of Sethria says: Take the other to the Firelands Hatchery! Soon we will possess the skies.
Sethria says: This is our mountain now, mortal. Begone!
Sethria says: You are little more than insects! When I am done with you, I will burn your shrine to the ground!
Druid of the Talon says: We're coming, <name>! Keep her occupied while we loop around for an attack. Sethria says: You buy time... but not your lives... my hatchery remains...