Sane Mage hints us he is both- Mad Mage & Raging Mage.
Sane Mage: I AM THE... I mean... I am - what is a mage, if he is not mad? If he isn't... raging? I am... I am just sane. A sane mage. 1
Also upon completing Elementalist Outfit Quest (bringing Elemental Spikes as a proof of defeating Raging Mage), The Broken servant Sentry says
Broken Servant Sentry: The Master is-is-is-is de-ad. Plea*chrrrrchk*se. Be. In. Mourning. 2
From the teaser:
He created Uniweel, opened portal to Yielothaxes. He also helped his brother Lloyd to build his "Fortress".
Another, speculative and possibly a theories are
- Since many items from Yielotaxes are used with Shimmer Swimmer he might have connection to Lake Equivocolao. Also the Ring of Ending recently become a "Prime Artefact", so possibly he had/will have even bigger role in the game.
- He is connected to Panestrider and Plangezers. His Reflection appears during Opticording Sphere Quest.3. So he might be involved in creation of the Supreme Cube and entering other dimensions.
Their Master's Voice 4
A magical tower has appeared east of Edron out of the blue. A mad mage without any connections to the Academy has created this building and conducted several experiments there - some of which failed quite horribly. The mage is nowhere to be seen but the lower dungeon is filled with his servants who might carry some slime mould which you should pay attention to if you would like to get the elementalist outfit.
Killing all servants will provoke their master to show himself and you will be able to fight him. However, if all servants are dead, a dangerous slime fungus will slowly start to spread in the tower and can only be removed with the help of the slime gobbler. The servants and the mad mage will only reappear if all the slime fungus is gone.
The Mage's Tower 5
South-west of Zao Steppe, a vicious raging mage has created a dimensional portal on top of a tower in order to access the energy buzzing there. The wizard himself is keeping the dimensional rift open. So as long as he is alive you are able to enter Another Dimension where you will encounter yielothax, strange and tough creatures with rare loot. Also, you might come across the Uniwheel, a very specific mount.
Killing yielothax will also weaken the raging mage and if a certain number of them has been slain, the raging mage can be defeated and you can get your hands on his unique loot. However, the dimensional rift will break down soon after killing the mage and players will have to leave the dimension until the raging mage will return and restore the portal.
Btw. Please note that last Part of the Diary, says the Lloyd used energy from his Fortress- which was created by previous occupants. It is not Mad Mage power/expertise.