SPOILERS

One who spoils; a plunderer; a pillager; a robber; a despoiler.

One who corrupts, mars, or renders useless.

A document, review or comment that discloses the ending or some key surprise or twist in a story, or the internal rules controlling the behaviour of a video game, etc.

To mark (a document or message) with a spoiler warning, to prevent readers from accidentally learning details they would prefer not to know.

To tell (a person) details of how a story ends etc.

"I've been spoilered, so I doubt I'll be able to enjoy the final episode."