The two most thorough compendiums of deity and domain information are imarvintpa’s Deity Search and Curmudgeon’s Cleric Domain List. Neither lists anything with “xv” or “iya.” This strongly suggests that the god is never defined in 3rd edition, as both sources are extremely comprehensive.
On the off-chance that either had skipped web material, I also checked a Google search of
site:archive.wizards.com "iyachtu xvim"
This resulted in only three hits: the Class Chronicles article you already mentioned, this Monsters of Faerûn web enhancement, which only mentions him in relation to some monster in that book (tyrantfog), and a Magic Books of Faerûn article, “Kaupaer’s Quick Book (Spells for the Hastiest Mage)”, which mentions that Teresken, the last known bearer of Kaupaer’s reflexive strike, “was disappointed with Iyachtu Xvim's attempts to claim Bane's legacy.” So neither of those help.
A search for just “xvim” reveals little more: a “beast of Xvim,” a passing mention of a “Fzoul” who became “Chosen of Xvim (and then Chosen of Bane),” the bearer of a cursed dagger being driven mad and running straight into a fight between Cyric and Xvim, a web enhancement promising that Cloak and Dagger (a 2nd-edition book) included a full description of Fzoul’s becoming the Chosen of Xvim, and that’s it.
And a search for just “iyachtu” revealed that the term is never used except in the phrase “iyachtu xvim.”
So, being out of any kind of official rules behind this, I tried the lore. Iyachtu Xvim was Bane’s son, and kind of proxy god while Bane was dead, used to revive Bane. Fzoul, who Monsters of Faerûn claims is the “Chosen of Iyachtu Xvim,” is nonetheless still listed as a cleric of Bane in Forgotten Realms Campaign Setting. According to the novels, he was a cleric of Bane, and then (insincerely) of Cyric, and then of Iyachtu Xvim, and then of Bane once again. It is possible that Forgotten Realms Campaign Setting and Monsters of Faerûn are just referring to him at different times, but considering Iyachtu Xvim’s close association with Bane and Bane’s divinity, it’s possible that he was simply carrying on for Bane during the Time of Troubles. So he may have simply offered Bane’s domains (Destruction, Envy, Evil, Hatred, Law, Pride, Tyranny, Wrath).
I also spoke with afroakuma, noted expert on D&D lore. His suggestions for Iyachtu Xvim’s domains were Evil, Hatred, Law, Strength, maybe Domination and/or Suffering too. Hatred would have been the important one.
Notably, of these options, only Domination would introduce a new option to Godsblood Spelltheft, and that was one of the weakest suggestions, so maybe it doesn’t really matter.