What shocks me about King's Tide, and why it ties with Hollow Mind as the show's darkest episode in my book, is how EVERYTHING that could possibly go wrong for the heroes DOES, and to a degree never before seen in an animated TV series as of its original air date.
Eda tries to sabotage the Draining Spell? She gets caught red-handed and loses that same hand as her lover seemingly dies in her arms, all while she, Lilith, and Hooty can only helplessly watch.
Luz intends to warn Eda and stop Belos herself? She fails miserably to do either, damn near gets the entire Hexsquad (including herself) killed trying, and loses the Clawthornes to the Collector, with her eyebrow being permanently scarred in the process. By the end, her four best friends, foster sister-to-be, mother, still-inactive palisman, and Eda's Grudgby jacket are literally all she has left.
Amity, Willow, Gus, and Hunter try to help Luz? They just get the ever-living crap beaten out of them alongside her before being run out of their homeworld on rails and stranded on Earth, effectively orphaning them.
King frees the Collector to stop the Draining Spell? He succeeds, but his plan works too well and causes another apocalypse, with him ultimately being forced to sign his own freedom away and give Luz the boot to save her.
That's not even getting into the apocalyptic and visceral imagery; the Draining Spell itself, Belos' transformation into his lich form, King's skull being cracked, the Hexsquad's injuries and ruined clothes by the time Belos is done with them, the thunderstorm the kids show up on Earth amid, that creepy-as-hell final shot where Belos' sentient slime closes the inert portal door at the end of the credits—all of it.
All in all, King's Tide is quite possibly THE bleakest, scariest, and most depressing season finale in ANY cartoon show EVER.