Two known spawn locations (PH "a mature icewing"):
Buffs itself with "Shield of Scales" every 30 seconds.
20 seconds into the fight (and every 40 seconds after), she emotes that she's laying an egg. Eight seconds after this emote, an egg spawns at the location she was standing when she made the emote. Anyone within a close range of the egg will cause it to hatch, spawning an add.