From Heroes of Might and Magic I to V!

Well, the Ironfists left their homeland, Enroth, and left Humphrey in charge... leaving a mere boy behind... that's what they got for it...
There was probably complete chaos in Enroth by that time.

So, Archibald teleports across many miles...))
Or his Angels saved him (and he hit his head during the rescue and developed amnesia)... he's sitting somewhere in Axeoth and is bewildered, "Where am I? Who am I?"

Enroth is also gone. Nikolai is in Axeoth. The Ironfists were also in Enroth before the Reckoning.

Added 39 minutes later

In Heroes 4, the description of Gem says that she drank water from a magical spring and didn't age for about 80 years, and after the Reckoning, she lost her immortality.

Gem was not immortal. The spring simply extended her life and greatly slowed down her aging during that time. A small spoiler: in Regeneration, we will say more about this spring and how it is connected to Alamar (how he didn't age) and Gem.

Why is Crag Hack consistently depicted wearing a horned helmet? ))

(I don't know what he looks like in Might and Magic 7) but I have a feeling he looks very similar.

In M&M3 and 7, Crag Hack and Varna have red hair and beards. Crag Hack from Enroth does not have red hair. And yes, as Mugais said, he looks like a dwarf. In M&M3, he is originally a dwarf. But in M&M7, since the race of the characters in M&M3 is not tied to the plot, he was made a human, and Resurrectra was made an elf. The Z.I.P. are equal to the Ancients, by the way, but this cannot be said about Crag Hack from SoD, and Crag Hack from Varna could not have been in Enroth during SoD (Demilich made a mistake, as the Z.I.P. arrived in Enroth around 1167). Apparently, one of the purposes of SoD in NWC's plans was to confirm that Crag Hack and Sandro from Enroth and Crag Hack from Varna and Sandro from Xeen are different characters, respectively. True, in SoD, they introduced a namesake of Etric the Mad - Etric the Warlock and former teacher of the Enrothian Sandro. And besides the 2 Etrics, 2 Hacks, and 2 Sandros, there are no other namesakes, except for Rolands (but even without the expansions, it is obvious that these are different people). Krodo, Zama, Zoma, etc. were not in Enroth. From Varna 4 (Varna from M&M1, intended for Enroth), Agar, who later became a lich, and Alamar, a descendant of the former ruler of Varna (one of the Alamars from his lineage was an opponent of Morglin Ironfist in the struggle for power on the continent of Enroth and a supporter of Archibald (that very "un-aging" one)), moved, and from Kron, the descendants of Calon and Slayer, who were opponents of Morglin Ironfist, moved. Characters with the same names are amusing moments, not inconsistencies. The inconsistencies are mainly in the Chronicles.