Both Prince Magnus and the Emperor mentioned in HeroQuest are one and the same.
AND are the same Emperor Magnus elected in 2304 in the Warhammer world.
In Warhammer, the last current emperor Karl Franz was the Elector Count of Reikland, Prince of Altdorf, and the Emperor of the realm maintaining all titles concurrently.
Prince Magnus from HeroQuest and Warhammer would be the Prince of Nuln, the same individual known as Magnus the Pious, a noble of Nuln, who repelled the largest Chaos invasion known as the Great War against Chaos in 2302-2303 and became Emperor in 2304 as the people demanded it. During his reign he spent much effort to eradicate remaining Chaos warbands throughout his lands, "the servants of Chaos were hunted down in the forests, and many wild and long-abandoned lands were re-settled." This after a lawless period known as the time of three emperors in 1979 where many claimants to the throne disrupted the succession of emperors for the next 400 years allowing Chaos to corrupt mens' hearts and infest the land.
Mentor is simply a good wizard living in the empire (long before the colleges of magic were established and magic was officially authorized) whose pupil Zargon/Morcar rebelled and became a Chaos Warlock possibly during the chaos incursion, and his pupil turned nemesis was still commanding a Chaos warband in the aftermath of the Great War. A minor albeit powerful warlord, not the leader of the entire Chaos army (who was defeated by Magnus at Praag, in Kislev) or particularly noteworthy to history in the vaccum right after the Great War. Much of the "HeroQuest can't be Warhammer" position stems from the lack of mention or scant mention of the HeroQuest baddie Morcar/Zargon in Warhammer history which seems incongruous but fits right in line with all the other mighty warlords and generals, victors of countless battles who scarcely recieved a line of note in Warhammer history.
HeroQuest fits the Warhammer timelime right after the Great Chaos war in the early 2300s which coincides with the then current in-game time of the GW publications of 3rd edition.
Orcs and Goblins were part of Chaos armies in the 3rd edition of Warhammer when HeroQuest and Battlemasters were released by Milton Bradley in conjunction with Games Workshop were in 1989-1990 (Chaos Goblins getting their own specific entry and stats much like the discontinued Chaos Dwarfs) and fimirs were also included in that edition. Any Warhammer army could take contingents of allies as well in 3rd edition, so it wouldn't have been at all unusual to see Chaos Warriors fighting alongside Goblins, orcs, fimir, or undead. Gargoyles are living statues and what would be more terrifying than a living statue of a Chaos Bloodthirster?
HeroQuest is set in the period right after Magnus of Nuln was elected emperor and then spent his remaining reign trying to eradicate remaining Chaos pockets in his realm.
The product lifespan of HeroQuest spanned the transition to 4th edition Warhammer and the beginning of army books which consolidated orcs and goblins, Undead, and Chaos into their own separate army books rather than the sole Warhammer armies book of 3rd edition making orcs and goblins fighting under the banner of Chaos unusal or considered uneasy allies at best. Fimir were excluded from 4th edition Warhammer on with only a few obscure mentions in later editions. At the same time Advanced HeroQuest with it's inclusion of Skaven and the WhiteDwarf HeroQuest quests were produced further tying HeroQuest to Warhammer.
HeroQuest is and was always intended to be part of the Warhammer world setting with three published maps of the old world published in HeroQuest products themselves and numerous references to places and people mentioned throughout.
I own all the HeroQuest products (except for the sticker book and a few puzzles), all Warhammer fantasy products from 3rd edition to the final 8th edition, several supplimental products such as Uniforms & Heraldry of the empire and the World of Warhammer, as well as a complete Warhammer fantasy roleplay 2nd edition set.
I also own all of the Battlemasters game, the only other game made by both Milton Bradley and GamesWorkshop which also has numerous references to the Warhammer world.
Everything I have read in all of the Warhammer products confirms without any doubt that HeroQuest is fully compatible and in fact set in that world, with only a few minor place name discrepancies or a handful of new monsters to differentiate the two. Medieval maps were notoriously inaccurate in the real world.
All of the arguments against seem to come from folks who do not have access to many Warhammer materials and thus assume there are major differences or incompatible timelines, even going so far as to say it is an alternate reality based on but different than Warhammer, which is far from what the evidence actually shows.
Warhammer, Battlemasters, Advanced HeroQuest and HeroQuest are set in the same world in the official materials published and that is that.
Now just like Dungeons & Dragons is each own dungeon master's universe to change as they see fit even if they are using a published campaign setting, the same is also true of HeroQuest.
I'm a conformist, my HeroQuest is set in the Warhammer world setting as is presented in the official products, perhaps yours isn't; it can be an alternate timeline for you or even an entirely different world as you wish.
