Amplify was included in the Legions set, part of the Onslaught block.

702.35. Amplify

• 702.35a. Amplify is a static ability. "Amplify N" means "As this object comes into play, reveal any number of cards from your hand that share a creature type with it. This permanent comes into play with N +1/+1 counters on it for each card revealed this way. You can't reveal this card or any other cards that are coming into play at the same time as this card."
• 702.35b. If a creature has multiple instances of amplify, each one works separately.

Rules Translation

• 702.35a. - This is an always active ability that only occurs when this object comes into play. So it is not a triggered or activated ability. When it comes into play, the caster may reveal any number of cards from their hand that share a creature type with it. This object then comes into play with that many +1/+1 counters on it.
• 702.35b. - Amplify stacks. So if you played a card which was somehow given a second set of Amplify, then you could reveal your hand twice and it comes into play with twice as many counters.