Er, contextually The Dark Lord of the Dead who tells Arthas to kill Mal'ganis is obviously Ner'zhul the Lich King. There would be no reason for Zovaal to tell Arthas to kill his own Nathrezim agent.
You have to realize that the Warcraft 3 story was written at a time when Shadowlands was never even invented or conceptually planned, and that the story of Warcraft 3 was meant to be self contained to the characters within it. Zovaal was never created back then, he is an invention of today, a complete shoehorning and retcon of past events. Contextually, the Dark Lord mentioned in the Warcraft 3 campaign only makes sense to be Ner'zhul, who was actively defying his Legion masters and wished to use Arthas to free himself from his Nathrezim jailors.
I wouldn't put much effort in trying to fit Zovaal into Warcraft 3 dialogue. It wasn't written with him in mind, and there's really no point in trying to string it into the Warcraft 3 plot circa 2001. We have to be mindful that Zovaal is literally a creation of now, that hasn't actually been seeded but rather shoehorned where best fit. It's like implying Draenei were always Blue Space Goats was hinted at in Warcraft 3, when it's the opposite and the old lore was retconned to connect two completely unrelated races to each other. The Warcraft 3 Outland Draenei had nothing to do with the Eredar, and all the lore that exists today is a retcon of past events with no actual connection in the Warcraft 3 game or storyline. Reforged was originally planned to add in a bunch of WoW lore in where it didn't exist before, but even that never happened since they scrapped the Remade campaign and just kept the original one, voices and all.
We have to be clear of terminology here. Zovaal is a new character in the context of Blizzard writers progressing the lore. He is not a character that had ever been seeded into the story before Shadowlands. Even Sylvanas' short story involving her dying had absolutely no hint or mention of the Shadowlands, and everything in the lore about Zovaal is newly built to support the current storyline, retcons and ancient backstory included. Of course he is 100% canonized as a lore figure who has existed for eras and is responsible for many of the events in Warcraft history, but that doesn't make him any less regarded as a completely new creation that has been inserted into the backstory. The Dark Lord that Arthas hears
in the Warcraft 3 game is not Zovaal, even with the retcons in mind. It was still Ner'zhul.
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https://wowwiki-archive.fandom.com/w...(Warcraft_III)
'Dark Lord of the Dead' is even directly linked to Lich King Ner'zhul in the WoWpedia entry. It was never Sargeras, and definitely not Zovaal.