https://meta.discourse.org/t/discourse-spoiler-alert/12650/
Spoiler plugin for Discourse highly inspired by the spoiler-alert jQuery plugin.
In your posts, surround text or images with [spoiler] ... [/spoiler].
For example:
I watched the murder mystery on TV last night. [spoiler]The butler did it[/spoiler].
This plugin has now been bundled into Discourse core.