Omnipresent Impostor’s second ability is a characteristic-defining ability that functions in all zones as well as outside the game.

Omnipresent Impostor has the names of all cards, including the back faces of double-faced cards. Notably, it doesn’t have the names of tokens, such as Zombie Token, Blood Token, and similar.

Omnipresent Impostor has the basic supertype, and you can play as many of them as you want in your Constructed decks. These two statements may or may not be related.

Each time you search your deck for one or more cards, you can find Omnipresent Impostor in place of one of those cards. If you search your deck for more than one card, you can replace each of those cards with an Omnipresent Impostor as long as there are enough Omnipresent Impostors in your deck. Continue resolving the search effect with Omnipresent Impostor in place of whatever card it replaced. For example, Sakura-Tribe Elder’s activated ability lets you put an Omnipresent Impostor onto the battlefield tapped, while Goblin Engineer’s enters ability lets you put an Omnipresent Impostor into your graveyard.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.