This forum doesn't support invisible text, but the effect can be simulated but changing the font color to white inside a quote box, like so.
Highlight to read: spoilers go here.
This should be used sparingly, if you've got a back-and-forth discussion going on in that format, it's best to take it to a spoiler thread.
If you wish to start a new thread to discuss a spoilery topic that might be too much of a tangent to include in the main thread, the thread title must be VERY VAGUE. Don't say the name of the character who died, or even that someone did die, or that a character made a surprise appearance.