There are some major mistakes in the script:
Zehir: Oh Biaraaaa. It's payback time.
This line is spoken by Raelag
Godric: Duncan... has been arrested for treason-
This line is spoken by Freyda
Leanna: Enough! Let the work begun by the Seventh Dragon be finished, here today, by my hand.
This line is spoken by Sareth
In their place, the name of Mal-Beleth, the new demon sovereign, appears. [...] Mal–Beleth is murdered by Kha-Beleth… [...] Some proof of the death of Mal-beleth: Ur-Vesphaal is the former demon capital...
Mal-Beleth never existed in Ashan, it was just early version of Kha-Beleth's name. Kha-Beleth replaced the Six Demon Overlords by himself, there wasn't any single Demon Sovereign besides him. He even has the armors of the Demon Overlords as trophies in his palace, you know.
This scenario doesn't mention anyone's death even in its dialogues - it's just the description... Also, town biographies from Heroes V are hardly the most decisive canon.
As I see it – Isabel had enough and was completely tired and burned out (she lost Nicolai and then damned his soul by her pact with Markal, then she break down and Markal started bloody war, all because of her weakness – and then Biara captured her, Kha-Beleth raped her and she gave him a child – Dark Messiah – after a few days of magically accelerated pregnancy). She just want to forget all this madness and rest for a while – and here was Raelag, who love her and just wanted to protect her!. They most probably travelled for some time together (Yg Chall is near Griffin Empire) so they talked and after some time, she started to like him and want to spend some time with him.
Isabel told Godric to go back to Griffin Empire alone, and that she will come later, after some rest with Raelag . Godric started his long way to the Griffin Empire, Isabel and Raelag reached Yg Chall – Raelag left his armies and disappeared with Isabel to his secret home.
On the way, some demonic cultist have stolen from him Heart of the Griffin (Isabel most likely gave it to him to take it back to Hall of Heroes), which was used to bring Biara back on Ashan (how???). She masked herself as Isabel, told Godric that she has changed her mind, and they travelled together back to to Griffin Empire…
From official story – Chronicles of Ashan - The Demon Sovereign, the Prophecy of Sar-Shazzar, and the Dark Messiah
By Jeff Spock, Writer, and Erwan Le Breton, World Coordinator
King Alexei's soul is stolen (well – Heart of the Griffin, stolen from Godric who is on his way back to Empire, I think) and in 971 YSD (Just after end of HOMAM V) its power is used to bring a succubus to Ashan; a succubus named Biara who arrives at a moment when she can take the place of the true Isabel (As I see it, she just came to Godric and told him that she has done her part with Raelag) The real queen remains lost in the wastes of Sheogh with her protector, Raelag... (Sheogh?? Zehir teleported them all to Silver Cities!! –They are in Yg-Chall!!)
Not exactly - the Isabel rescued from Sheogh, taken to the Silver Cities and then, to the Griffin Empire was Biara all along - the real Isabel remained in Sheogh and Raelag, realizing that Isabel was replaced, returned there in order to get her back. It was lengthy process, which is why they made it to the Griffin Empire so late.