I'm not sure when it comes to triggering events. My pages are mostly marked M for blood spilled during fighting and for injuries - which isn't as much blood or gore as the warning might make you think there is. But I keep it there just in case.
And my main concern is people who can't access M rated pages (I read you can't see them at all if you're not logged in). So I can kind of summarize it a bit more descriptive if I want to. But I mostly plan to just say "After a brief fight in which character A killed one of his attackers, he was overpowered, injured and captured."
For something that could be triggering, I would maybe keep it simple? Like "the events lead X to self harm". I think it's fine if it's more similar to a trigger warning and doesn't go into details. X's feelings seem like something you could probably return to in the story later, on pages that don't show the self-harm scene but just talk about it. That way those that are triggered by X's mental state can see the warning and skip the page. And those that skipped the previous pages because of the act itself but don't mind reading about X's state of mind can now go deeper into the story.