The War Within arrives this year—promising to finally address Battle for Azeroth's biggest dangling plot thread: the giant bloody sword that's sticking out of the planet. We know this because of some subtle hints from the expansion's cinematic trailer, like when the camera pans to the giant bloody sword. The one that's sticking out of the planet.
In case you're out of the loop, the sword was put there by Sargeras the Dark Titan, leader of the Burning Legion, when he was finally slain in the Legion expansion. This caused Azeroth to bleed a material called Azerite—kicking off the events of Battle for Azeroth and yet another unpopular borrowed power system (giving players new toys only to yoink them away at the end of the expansion).
Only, everyone kinda forgot about it during Shadowlands, and it's been collecting dust ever since. Like a garish galactic hood ornament.
An easter egg discovered by Twitter user Korrin/Hammerman753 (thanks, WoWHead) double-confirms that the blade will be important again. If you trek all the way out to Silithus with some certain trading post items transmogged, then chat to a lonely NPC in the middle of the desert, you get some special dialogue.
The items themselves come from the Cosmic Weapons Cache, which was available in November. The NPC is called the Twilight Hermit—a character plagued by visions that stopped when Sargeras did the big stab.
If you approach him with Gorridar or Seetharas—weapons that look a lot like the big, looming sword not that far away from the both of you—it causes him to ask:
"Why did [Sargeras] strike this place? He was far too masterful a tactician to choose this target by chance. His crusade sought to eradicate all life from existence. Do you think his final act before being captured by the Pantheon would not be in service to that goal?"
Meanwhile, approaching him with either Taesavir or M'thorus (daggers of titan make) will cause him to say: "Even the Dark Titan had a pattern to his actions. A cold logic intended to impose his perfect vision, and the willingness to eradicate existence if he could not achieve his ends. Do you think the rest of his kind are any different? Only in method, not in goal."
Lastly, Voidsong is a staff that used to belong to Xal'atath, an ancient dagger now possessing the body of an elf—a dread weapon from the time of the Old Gods. We know she's going to be significant, since she showed up towards the end of The War Within's feature overview.
"There was a time when this whole world was ruled by the Black Empire. It was beautiful. The masters bestowed gifts upon their faithful. That stave was one such instrument. It was given to the Harbinger so that one day she might... she might… What? No! I was not going to tell them! I... I…" He then goes quiet, as if "chastened by some unseen presence."
So—we can gather a few key things from this. First off, Sargeras stabbing the earth was likely part of a bigger plan, rather than a fit of gamer rage. Thrall says as much during that fancy cinematic trailer, that it was "aimed at something." While Shadowlands and Dragonflight distracted Azeroth at-large from the big bloody sword, Sargeras' posthumous game of 4D chess was likely ticking over.
Secondly, Xal'atath—or her masters—are up to no good. The hermit was definitely talking to someone, and was gagged before he could snitch. As to what their plans are? Don't worry about it, it's probably fine. Ultimately, this is a pretty cool easter egg to plonk in there, and it's a sign that WoW's story will be less distracted in the future. We've been promised a three-expansion long arc, after all.
