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Some of the references to the original Might and Magic universe in the Campaigns.
Griffin's Doom (first mission for Slava):
Pleasant Valley - the name of the region where Sorpigal is located, one of the five main cities of VARN 4 and the starting location of MM6. Also, Paradise Valley was sometimes referred to as such - a region on the western coast of Enroth, where the Kreegans landed in the north during the Night of Falling Stars. There is an obvious reference to this in the mission's plot: in the backstory of MM6, King Roland set out with an army to Paradise Valley to investigate rumors of devils and eliminate any potential threat from them.
The Emperor's Will (second mission for Slava):
Blackfang was the name of the castle of the vampire lord, Count Blackfang, on the Dark Side of Xeen, which plays a key role in the plot of Might and Magic V: Darkside of Xeen.
Valeska - the name of an Erathian Crossbowman and Knight, one of the recruitable heroes in Heroes of Might and Magic III.
No Place for Orc Friends (first mission for Sandor):
The fortress of the Wolf Duchy, considered impregnable and guarding the port by the Jade Ocean, is called Kriegspire. This was the name of a castle in Enroth that served as a center for magical research. Minotaurs were created during experiments at Kriegspire.
Marriage Between Heaven and Hell (fourth mission for Kirill):
One of the cities in the vicinity of Portmeiron is called Sotbigal-by-Sea. In Might and Magic Book One: Secret of the Inner Sanctum, there was Sorpigal - one of the five main cities of VARN 4 and the starting location of the game. Also, in the original universe, there was New Sorpigal - one of the six main cities of the Kingdom of Enroth and the starting location of Might and Magic VI: The Mandate of Heaven.
Another city by the coast is called Baywatch. This was the name of one of the five main cities of the Isles of Terra, located on the shore of the Great Sea.
One of the signs contains a bounty notice for the head or capture of the barbarian Craig Hack, who became a pirate. This name was borne by two barbarians in the old universe: a dwarf who is one of the Ultimate Adventurers, great heroes from Terra chosen by the Ancients, and a human from Enroth, a former fortune hunter. The former was one of the key characters in Might and Magic III: Isles of Terra and Might and Magic VII: For Blood and Honor; the latter appeared in Heroes of Might and Magic III: The Shadow of Death, had a cameo in Dragon Slayer, and was a secondary hero in Heroes of Might and Magic II: Succession Wars and Heroes of Might and Magic IV.
Death-in-Life and Life-in-Death (fourth mission for Irina):
Portsmith, a trading city of the Wolf Duchy (also appearing in Sandor's first mission), is named after one of the five central cities of VARN 4. On the sign pointing to Portsmith, someone wrote "Demon lovers!" In MM1, Portsmith was ruled by the Succubus Queen. This city also figures in Sandor's first mission.
Dawn Everywhere (third mission for Anastasia):
The staff of Sandor, a former follower of Belket, which Anastasia's Necromancers use to recruit Lamassu into their army. Sandor is the name of two liches from the original universe games (one from Xeen, the other from Enroth), who played a significant role in the plots of World of Xeen (Sandor of Xeen), The Shadow of Death, and Might and Magic VIII: Day of the Destroyer (Sandor of Enroth).
Also, one of the Necropolis is called Abel-Vertigo (I don't remember the exact spelling of the first word). Vertigo is the name of one of the five main cities on the light side of Xeen, from where the future Chosen began their journey to defeat Lord Xeen - the starting location of Might and Magic IV: Clouds of Xeen.
Spider's Trick (fourth mission for Anastasia):
Near the starting location of the game, there is a sign containing an invitation to join the Cult of Meow. This is a humorous reference to the tradition of the Might and Magic series where (starting from the third part) cults with names similar to animal species and the sounds they make played a significant role in the plot: the necromancer cult Moo on Terra (MM3), Yak on Xeen, the orc religion Bark, and the peaceful church of Mok on the Dark Side of Xeen. The order founded by the Kreegans to eliminate all potential threats to their invasion in the Kingdom of Enroth was called Baa (present in the sixth and seventh parts). In MM8, there is a Temple of Eep in Jadame, whose followers voluntarily turn into wererats. In Chedane (Might and Magic IX), there is the cult of Honk, who worshiped the goddess Fre's pet goose, calling it "The Great Honk".
Everything Mixed, Truth and Falsehood (second mission for Anton):
Lord Kilbourn, an ally of Anton and an assistant to the protagonist on the Path of Tears. Kilbourn is a noble surname from VARN 4. Two representatives of this dynasty were characters in the old universe games: one in MM1, who had to be found on a quest for Lord Ironfist. The other was one of Morglin Ironfist's supporters who fled with him through a portal to Enroth and appeared in Heroes 1-2 and the backstory of Might and Magic VI (he was killed by Kreegans while patrolling the outskirts of Blackshire).