Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
Before you can even embark on this Errand you first have to find Horace, the ghostly little boy who's been dogging Oliver's steps throughout his journey. Head to Ara Memoriae and check the northern pot . . .
. . . and . . . there he is. Looks a bit different, these days, doesn't he? He's still Horace, though, and he has a question for Oliver: he wants to know what a certain set of people from his kingdom wore, long ago. Look through the Wizard's Companion and you'll eventually puzzle out that they wore 'Wizard's Robes', thanks to letters in red in the item's description. Do this and Horace will give you the Astra spell, which is fantastic for crowd control.
Thanks to Oliver, Horace has remembered who he once was. More, Horace has remembered his whole history, and he wants to tell it to Oliver . . . in instalments. You'll have to follow the Sage of Ages around the world once again, visiting his various haunts and answering one more round of riddles in Great-Great-Grandsage. Intriguing.
Horace's first leg of this Errand is in the same place where it begins. He'll tell a bit of story, and he'll ask you to name a wand with which you're intimately familiar: 'Mornstar'. That's the answer. He'll then zip off and tell you to follow.
The Tombstone Trail
Horace wants you to meet him where you found Mornstar, which, naturally, is at the very end of the Tombstone Trail. He tells of three great servants of the Wizard King in the days of old, and he asks if you remember the name of the serpent lord. Of course you do—it's 'Aapep'.
The Vault of Tears
Now Horace wants to talk to you where you beat aforementioned serpent lord. Run to Aapep's room in the Vault and you'll find Horace still reminiscing over the past. His next riddle involves looking at the very last page of the Wizard's Companion and reading the first letter of the poem (red text) from bottom to top. The answer is 'White Witch'.
The Glittering Grotto
Horace is headed to a place where you can see the past, it is reputed, and that place is Glittering Grotto. He's waiting in a room about halfway through the Grotto, where Oliver saw a vision of his mother. He wants to know what's written on the wand Pea gave you at the beginning of the game. Translate the letters on it and you'll get 'Bring Hope'.
The Ivory Tower
Your final destination is the council chamber in the Ivory Tower. Make your long and tired way back there to find Horace waiting near the doors to the final boss. He'll tell Oliver the last of his tale and ask what magical instruction manual he (Horace) wrote. Easy answer: 'Wizard's Companion'.
Completing this errand will earn you a Mirror of Truth, 10000 guilders, and new alchemical formulae on three awesome weapons for Oliver, Esther, and Swaine. (Who cares about Marcassin, he's a latecomer.) Thanks, Horace! It was fun chasing you around the world for the entire game!