This time Octo said pretty much what I was going to!
This season was a messy ride, but I think it stuck the landing. Biggest shame about it was that they basically did all that to retread the Rezo arc from the first season when there was other stories from the novels and manga they could have pulled from. Hell, there have been other shards of Shabranigdo in the story they could have used, but they gave a re-do to the one the anime already started with. (fun bit of trivia, I've since learned Zuuma was from the novels/manga, he originally appeared as one of Kanzel's servents sent to kill Lina during the Seiruun plot during the events of NEXT, and persisted for awhile after popping up to try and kill Lina and eventually fusing with another monster, like he does here, but in his original appearance the monster he fused with was the remains of Seygraam, and I can't help but think that would have been so much more interesting than the two forgettable mooks he fused with in this series)
I appreciate them threading the needle and creating this convoluted method to bring Rezo back and also accounting for Shabranigdo returning and having it make sense with what we know. We already know Rezo is the sort to take extreme precautions, see his Plan B for killing Shabranigdo by absorbing the original Zannafar in Sairaag, so having a Plan C just in case he ended up dead anyway with the Hellmaster's Jar is totally in character. But once again the fact that he didn't know Shabranigdo was sealed within his soul came back to bite him, and when his soul was sent to the Jar he had some unexpected company.
But if course Rezo is still the biggest jerk around and instead of having the jar smashed he decides to take Pokota's body anyway just because he wants to see, knowing that he can (probably) leave Shabranigdo up to Lina once more.
Literally retreading the showdown with Shabranigdo and ending it with Lina casting the Giga Slave (and turning her hair grey again), only this time with her relying on her amulets for the magical amplification instead of the sword of light, felt like a bit of a cop-out, but I won't complain too much. The show did a lot to win me over by playing Give A Reason during the final battle, after all.
Ultimately the series ends with the usual status quo. Amelia going back to Seiruun to do Princess things. Zelgadis going off on his own once again. Lina and Gourry running off to find a new sword for the latter. And Xellos teleporting away with his usual wry smirk. Nothing is changed! Everything is the same as always!
Honestly remembering to stick Prince Phil, Sylphiel, and Wiser in the final credits but giving them no lines just feels like a slap in the face. You could have been using these characters the entire time in Evolution-R! Why remind us of them now?
But this is the end of the series, for now. It could always come back again, I suppose, and there actually have been some new Slayers stories published as recently as a couple years ago, so who knows?