This quest, one of the best ever created, has immense mysteries that have been unveiled over the course of different updates, and will also be the target of some hints from Tibialight.
Many believe that to make the supreme cube it is necessary to face all the known challenges of the quest, but not quite. To be possible, there is only a need for the platform present on Gray island to be visible to the character (photo below).
And for the platform to be visible, a minimum number of missions is necessary, we will see here which are the essential ones in order:
Mysterious Ornate Chest in Ab'dendriel.
Use the opticording sphere in Venore (3x).
Use the opticording sphere in the Horestis Tomb.
Use the opticording sphere in the Drowned Library (Deeplings).
Use the opticording sphere in the Mushroom Garden (Gnomebase).
Use the opticording sphere in Ab'dendriel's temple.
Opticorder Forge in Kazordoon.
Mysterious Ornate Chest in Ab'dendriel [again] to obtain the Mysterious Metal Egg.
Otherworld (vortex) in Kazordoon [To obtain a Complete Opticording Sphere]
Krailos [Use the Opticording analyser].
After these steps, the character on reaching Gray Island starts to see the platform and it is possible to enter the Supreme Cube's assembling room!
The other missions known in this quest, only exist to obtain items to enable the cube to be made, and are not requirements.
The cube [Obtained in the Last Planegazer's fight].
Missing Piece of the Cube [Obtained in Edron, on Galthen's Grave, near the Falcon Bastion's Edron entrance].
These items above are necessary for the making of the supreme cube, but if a character receives it from someone else, or buys it, it will be of the same use, without the need to carry out missions other than Krailos' mission 5.
Finally, there was no mention of a boss within this quest: "An Observer Eye".
This boss is not a requirement for any part of the quest, nor does it seem to be before any other stage, its only drop: Lightsphere still has a mysterious use, but certainly in the quest chronology, it presents itself after defeating Planestrider.