I just finished watching the first part of
Owarimonogatari by finishing up Episodes 7-12 which covers the arc
Shinobu Mail arc. Honestly, a lot of Owarimonogatari so far has been great, but hasn't reached the highs that occurred in Monogatari 2nd Season. However, this arc did just that.
This arc follows through the Shinobu Time arc back in Monogatari 2nd Season. This has to do with the task that Gaen asks Koyomi to help her with, which involves the help of Kanbaru. Both Koyomi and Kanbaru run into Shinobu's first disciple, Seishirou Shishirui. To put quite simply, if the cram school didn't get on fire, then both Koyomi and Kanbaru would have died. Seishirou requests the apparition sword which Shinobu currently posses. Of course, she does not seek to give that out to him.
The situation is odd concerning that Koyomi and Shinobu's pairing is still not linked up. Which puts them in a bind to get Gaen to restore their link, which requires fulfilling her request.
As we know from Kizumonogatari, we know Shinobu is an immortal vampire. Assuming she has enough of her powers. Since we know that Koyomi is an immortal vampire granted currently post Kizumonogatari his powers are much weaker. By that logic, this would imply that Seishirou is also an immortal vampire. One would assume that a vampire going out on a suicide by going into the sunlight would mark death. But consider that can you really die if you are immortal?
Eventually, the sun will fall as night occurs. Even though Seishirou is ash, that ash at a small rate will recover. Now, depending on whether he can avoid the sun or not, his recovery progress will be staled. Given that it took him 400 years to come back in a state of being, it's clear that it was a tough ride for him to come back
The most interesting thing which Gaen brings up is that all the apparitions that have occurred because of Koyomi, Shinobu and Seishirou. First the ash which Seishirou was a part of as he was recovering came back to this town and more specifically the shrine. Because the shrine couldn't hold Seishirou's presence, it collapsed. In which all the apparitions were released. Back in Bakemonogatari, Meme Oshino did put a talisman on the shrine, but that was a band-aid fix which Gaen mentions. It was him restoring balance, not actually addressing the issue, which is what Gaen wanted.
The problem with Seishirou is that he can use one of the abilities that Shinobu knows, which is Energy Drain. This is simply something that speeds up his recovery. Unlike the case with Shinobu where her power resides in Koyomi at this point when they are paired up, this isn't the case with Seishirou. Seishirou can attack humans and use Energy Drain to get more and more powerful. Basically, he would be approaching powers that are close to Heart-Under-Blade in her peak form. No surprise Gaen wants him killed.
Before the fight between Koyomi and Seishirou starts, we have an interesting verbal argument between Kanbaru and Shinobu. This is something that really highlights Kanbaru's character. Quite simply, Kanbaru's character is essentially devoted to emotion. It was highlighted in her arc in Hanamonogatari and you can even see how her character is pure emotion when she is with Koyomi during their banter. Ultimately, she is bold enough to say to Shinobu that you need to face your first disciple. It doesn't matter whether it is logical or not, you need to face your first disciple.
As Shinobu points out, it could be just a waste of her time since Koyomi is her current disciple. Kanbaru offers Shinobu a lesson that you have to deal with the demons you don't want to face. Seishirou was Shinobu's first disciple, and she was heartbroken when he attempted to commit suicide. While it is true that Seishirou wants to kill her and take back his apparition sword. At the same time, he could also desire to be her only apprentice. Whatever is the reason, Shinobu needs to encounter and give her own answer back to Seishirou.
The dilemma of logic vs emotion also affects Koyomi. Back in Monogatari 2nd Season in the Tsubasa Tiger arc, Hanekawa was facing off against the Tiger Apparition. Given Koyomi was about to fight Seishirou, he was left with a decision. He was the only one who could fight this battle for Shinobu. He decides to trust Hanekawa and Senjougahara that they will make it through alright. Koyomi unlike Shinobu is much more emotional. In this case, he makes the logical choice since he is the only one that can fight for Shinobu. But it shouldn't be misinterpreted that there are no emotions behind this decision as he cares about Shinobu a lot.
Also, for him to trust his girlfriend and Hanekawa is a big moment for Koyomi because he has a savior complex. For him to take a step back and trust that they will be alright is big because it signifies his trust in them. Which we know turned out well given the conclusion of the Tsubasa Tiger arc.
As the battle starts, Koyomi has an interesting trick up his sleeve. While logically, the contest between him and Seishirou is a matter of who picks up the apparition sword and kills the other. There was nothing in the conditions to solidify that you can't use other tools at your disposal. This is essentially what Koyomi does as he uses the talisman from the shrine to immobilize Seishirou. Since putting a talisman on a person is much faster than swinging a huge sword. With that, Koyomi seized his victory.
While Shinobu wanted Koyomi to fight his battle, she had the courage to face Seishirou one last time. You can tell with how her emotions flare up that she misses him and forgives him for what happened when they parted. She mentions that Koyomi is her partner now and offers a farewell to Seishirou that he wants. In which his suicide is now completed and he can rest in peace.
Now we come back to the end of Koyomimonogatari before Koyomi heads to the shrine where Gaen kills him. Currently, we have all the important events that have led up to this moment. We have spent a lot of time with Ougi Oshino who is intelligent, but very mysterious all the same. She knows a lot of what is happening. As Ononoki mentions, Koyomi should be careful about saying too much in front of her. Giving how Gaen acquires another apparition sword and how Ougi figured she would do that is interesting. Is there more to Gaen's logic than appears on the surface? Also, who were the group that was going after Kagenui?