It's as promised by the title. War in Underworld. Bad things are coming to a virtual world where the AI people have souls, where a brain-damaged Kirito has been trapped for subjective years. "Underworld" is a misleading, lurid name, but "war" is exactly what's about to happen.
It breaks down as follows:
The calm before the storm. We see the fallout from the first half of Alicization. The Dark Territory are gathering their armies, in an event that's been calmly and deliberately planned by the real world overseers. It's a load test. For the Underworld's people, though, it's a bit more than that. Kirito's just trashed their leader and Integrity Knights, who'd even at full strength probably couldn't have stopped a full Dark Territory invasion.
Some real world baddies also get into the action. One of them gets a focus episode to himself (ep.4) and bloody hell, he's evil. "Creepy and nasty" doesn't begin to cover it.
BIG FIGHTS. That's what war is, after all... but, yeah, big fights. Lots of 'em. Anime fans all want to see fight scenes, right? (At one point I wondered if I'd skipped something and was watching the wrong episode.) This could have been dull, but...
1. There's characterisation, awesome heroes and overlapping motives. Dark Emperor Vector only wants one thing and doesn't care about his subjects or lands. The good guys are capable of being mentally weak, or outright broken.
2. The real-world baddies can cheat. For them, the Underworld is just another computer game.
3. Kirito's friends from pre-Alicization story arcs get into the action too. This is surprisingly cool. After 100-odd episodes, Sword Art Online has notched up a lot of story weight and people we want to see. (This includes the dead ones.) What's more, the story's tone is so sincere and unblinking that it pulls it off. There's no irony. No self-awareness. No winking at the audience, or anything like that. When the show does a Big Damn Heroes moment, it's so wholehearted that it has power even if you only half-remember the characters who are returning.
(I should apologise, incidentally. I previously called the show humourless, but Alice vs. Asuna in ep.10 is mildly funny. Thinking about it, in fact, Alice has a claw-showing moment with Lady Fanatio in ep.5 too. It looks as if it's not hard for other women to make her bristle.)
4. There's actually quite a lot of plot.
The war's over, but the show's still got things to say. I loved the press conference debates in ep.22. Unexpected things happen, repeatedly. I'm not sure I like a certain big deletion in ep.22, mind you. It might have been better storytelling just to kill the character. However, it does give rise to yet another big surprise that could be fascinating to see play out.
(Yes, Sword Art Online will continue. You couldn't stop here.)
It's big, it's unblinking and, when required, it's intelligent. It's successfully doing "epic". I like the cast. Note Alice's anger in ep.15, for instance, or Kirito's fragility. After the preceding 73 episodes of trauma, it's going to take him most of the season to recover up to "wreck". It threatens to get a bit battle-dull about a third of the way through, but only a little and the show basically avoids that trap. This is a strong show that's grown from its early episodes into something impressive.
I'm looking forward to Sword Art Online: Progressive, a retelling of the early floors of the original Aincrad arc. Less so to the announced Netlix live-action adaptation, though...