Hello again Omeganian mate!
So the master is a necessary piece of the spell component/contingency?
Well, this is a blood spell.
'The' Devil or 'a' devil. I wonder would any Arch-duke suffice?
Difficult to say. There was no sign of such creatures. However, it might. You see, the basic structure of the magic there is:
1) Almost all magic in the world of living can only trandform and create (Additive).
2) Another type of magic exists, which can undo and destroy (Subtractive).
3) The ability to use that magic has been taken away from wizards 3000 years ago.
4) Spells composed of both types were shown to be truly epic.
5) the Subtractive type of magic belongs now mostly to the underworld (which is a powerful Negative Energy dimension there). Among the wizards, only the servants of the Keeper of the Underworld can use it (and the protagonist).
How did he win?
Not him. And the defeat was not by force
Did anyone actually enter this 'dimension' beyond the gate?
Not as far as I know. Surviving in other dimensions is pretty tricky there anyway.
The area within the boundary overlaps the real world with the Netherworld (Shadowfell?).
It seems to be negative energy.
Incidently, regarding the Gray Gripper creature, why couldn't they just slice the monster down to the limb rather than have to sever the limb entire? I mean I understand the idea that you couldn't pull it off, but unless the thing is nigh on indestructible, couldn't it just be chopped up itself?
Well, it is well armored, and creatures of the underworld, since they possess Subtractive Magic, can often negate Additive. In any event, a rather powerful sorceress armed with an axe had to use it to cut her own foot off.
I recommend you read some of those books (The Sword of Truth series). There are eleven of them, but after the fourth one or so, there seem to be quality problems.