Nulian the Stormwarden (click for map image) spawns in the middle of the island (PH "a storm champion").
Map points for "tempesttemple.txt" map files (for all Tempest Temple named map points, see this quest entry):
Casts "Magma Storm" every 45-60 seconds (DOT for 18,000 dmg/tick for 3 ticks).