"At the hour of her third death, she will usher in our coming."
So yeah, that's a big one.
The immediate person to jump to in this is Sylvanas Windrunner. This is a woman who has died multiple times and she's also currently Warchief of the Horde, not to mention that she has demonstrated, even after being made Warchief, a certain disregard for conventional morality (see Stormheim)
(Actually, there's a whole article to be written about how I wish Stormheim had been more different between Alliance and Horde than it actually wound up being, but we'll table that for now.)
Sylvanas ushering in - not necessarily intentionally - some great evil is not terribly hard to imagine happening.
But here's the thing. Sylvanas has already died three times.
The first time is the obvious one - when Arthas was invading Quel'thalas shortly after being turned by the Scourge, Sylvanas was Ranger-General (her older sister Alleria had disappeared into Outland and beyond at that point.) As the leader of the defenses, she was personally killed by Arthas and then personally raised as a Banshee (and she actually wound up possessing her own corpse, which... kind of amounts to being back in her body?)
Lots of stuff happened between then and her second death. In one of the Cataclysm-era Leader short stories, Sylvanas climbs to the top of the Frozen Throne to make sure Arthas is really gone. The good news is that he is, but the bad news is that there's a new Lich King sitting on the throne. In a fit of despair, she throws herself off the tower. She finds herself adrift in a cold dark place (and I think even might see Arthas' soul adrift there as well) but she is pulled back by the Val'kyr, who quit the Scourge to serve her.
The third death is in-game. If you finish the quests in Silverpine Forest as a Horde character, right after Crowley agrees to pull back to the Greymane Wall, the turncoat Lord Godfrey turns his coat again and shoots Sylvanas in the back of the head. Three of Sylvanas' seven Val'kyr sacrifice themselves to bring her back once more.
So unless the short story was invalidated, Sylvanas has already had three deaths.
So who has had two?
I honestly can't think of any, unless we're talking metaphorical deaths. Jaina is often presented in the fan community as a potential future villain (though I've held the stance that while I don't agree with what Jaina has been doing, there's nothing about her behavior to suggest that this is anything but pretty justified anger and mistrust motivating her.)
You might consider the destruction of Theramore as a kind of death for Jaina. Is there another one? There are certainly betrayals and traumas, but I don't know if anything else she has experienced would really be intense enough to call it a death, even in a metaphorical context.
As always, there's also the tin-foil hat idea of the Titan Azeroth being the one that they're referring to.
Here you could maybe think that the removal of Y'shaarj, which tore open the Well of Eternity, was something like a death.
And then the destruction of that well (which... wait, if it was a gaping wound and it was destroyed, then... oh crap) could mean that the Sundering was something like a second death for her.
Now the Old Gods aren't exactly trying to kill Azeroth. Their purpose is to corrupt her.
But the total corruption of Azeroth would certainly qualify as a metaphorical death. And indeed, the whole point of corrupting a Titan is to help the Old Gods open the way for the Void Lords to take over the universe, so...
You know, it was a tin-foil hat theory, but I actually think it's the theory that fits best.