I'm not new to this game, in fact I've been playing since the first day of the NA release and I didn't know what Grand Casters are? I found this is my servant list and didn't know who this was. Could someone please explain?
In the lore every class has one servant that is the definitive servant of that class. In this case, the Grand Caster is King Solomon. He's presented as being the primary antagonist in the main scenario at the end of Singularity 4. I should also note that he's not a summonable servant.
Note that the Grand title is a specialized type of servant container with restrictions on what Heroic Spirits can fill it. Apparently the qualifier for Grand Caster is the ability to foretell the future. The other container restrictions are unknown at this point in time
If you don't know who Solomon is, then you probably skipped the main story cutscenes. If you want to read them again you can go to My Room -> Singularity Log.
The part about Solomon is in the Fourth Singularity, Section 13. You can skip ahead until the background changes to the cave, if you want.
