Raging Mage spawns after the Energized Raging Mage has been killed.
This, however, will not happen until your server has killed 2000 Yielothaxes. This raid message will appear when he has been killed:
The remains of the Raging Mage are scattered on the floor of his Tower. The dimensional portal quakes.
Five minutes later the dimensional portal gets closed and this message appears:
With a great bang the dimensional portal in Zao collapsed and with it the connection to the other dimension shattered.
Raging Mage spawn occurs only if 2,000 Yielothaxes have been defeated. To have the right to face it the player must contribute to World Change by killing at least 250 Yielothax. The boss is active for 1 hour, disappearing if no one defeats him. Chance for looting elemental spike is 1,18%.