Narod’s ability doesn’t actually change any creature’s power. It changes only the amount of combat damage the creature assigns. All other rules and effects that check power and toughness use the real values, even if they cause damage “equal to a creature’s power” to be dealt.

If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.

Tokens that aren’t copies of other permanents will usually have a mana value of 0, making them very bad at dealing combat damage while Narod is around.

If there are multiple effects that attempt to modify how a creature assigns combat damage, that creature’s controller chooses which effect applies. For example, if Narod, the Beige Flower and Doran, the Siege Tower (“Each creature assigns combat damage equal to its toughness rather than its power”) are both on the battlefield, each time a creature would deal combat damage, that creature’s controller chooses whether it assigns combat damage equal to its mana value or its toughness.

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.