Complex sentence: The Sentry known as Robert Reynolds first appeared in his own limited series, in which he was marketed as a forgotten creation of Stan Lee, a falsehood that was eventually revealed to be a marketing ploy, and Robert Reynolds had been retconned into the Marvel universe.
Split into simpler sentences: The Sentry known as Robert Reynolds first appeared in his own limited series, where he was marketed as a forgotten creation of Stan Lee. This hoax was revealed to be a simple marketing ploy, and Robert Reynolds had been retconned into the Marvel universe.