It's an action adventure game whose emphasis is on combat, not a point-and-click adventure game whose emphasis is on puzzles. The gameplay and focus is entirely different, so no don't consider it a core title. Though I wouldn't call it periphery so much as simply a spin-off.
Again, your 'intepretation is off".... The emphasis was on adventure, with combat added. Much like Quest for Glory... But that's brought up over and over, in interviews, and on the KQ8 website, and the box even... Combat was an 'addition', not an 'emphasis'...
It was never meant to be a 'spin-off', and was always stated to be KQ8. It was stated as such on the website...
Only people who lie or are ignorant claim it was 'meant to be a spin-off'.
Again check out the thread; Never mind I'll repost her;
found one of the only captures of the old KQ8:MOE forums, and a capture of the main King's Quest forums on Sierra's defuct website. It's interesting to note that in the forums Sierra officially listed the game as King's Quest VIII: The Mask of Eternity.
This forum was up for a while it it was shut down in the mid 2000s, when Sierra closed. These archive captures are from about 2004.
King's Quest VIII: The Mask of Eternity at the Sierra Forums
King's Quest Sierra Forums
Also another detail I discovered was on the official KQ8 website;
King's Quest: Mask of Eternity website
Once may notice that the browser tab for the page is listed as KQ8: Mask of Eternity.
If you hover the cursor over the headers of each page, you get popup message like this;
If you go into some of the information on the website as well, there are references from Mark Seibert and links to interviews where he calls the game King's Quest 8 as well.
So Sierra was actively advertising the game as King's Quest 8 at the time.
Also an interesting bit of trivia is that in the game files there are two folder groups in each level folder, that are named ''8Bit'' and ''8Gui''. This is a nod to the game being KQ8. The first folder holds the 16-bit bitmap artwork textures, and the second folder holds the 'graphical user interface' files.
It's a bit a myth when fans lie (ya some fans are a bunch of liars and cheats) and make the false claim that "Sierra never considered this game "KQ8"". When in fact they were advertising it as KQ8 all over the place (and it even exists in the files).
To this day Roberta Williams and Ken Williams still refer to it as King's Quest 8/VIII.
It's interesting to note that the choice to leave off the number on boxart, is shared with other games produced between 1990 and 1998 such as The Beast Within: A Gabriel Knight Mystery (aka GK2), Quest For Glory: Shadows of Darkness (aka QFG4), The Lost Secret of the Rainforest (EcoQuest 2), Police Quest: Open Season (aka PQ4), Police Quest: SWAT (PQ5), Police Quest: SWAT 2 (PQ6), etc.