When the phase during which Runed Terror entered the battlefield ends, the next player in turn order will begin that same phase.

Steps in each phase play out as normal. For example, when you take your beginning phase, you’ll have an untap step, an upkeep step, and a draw step, and then the next player in turn order will take their beginning phase.

If there are multiple Runed Terrors on the battlefield, their abilities are mercifully redundant.

If an effect cares about whose turn it is, it’s the active player’s turn.

Effects that last “until end of turn” or apply “this turn” expire during the cleanup step of the player who was the active player when that effect began to apply.

If an ability allows you to do something on or during “each of your turns”, that permission resets after your ending phase. For example, if you control Muldrotha, the Gravetide (“During each of your turns, you may play a land and cast a permanent spell of each permanent type from your graveyard”) and you cast a creature spell from your graveyard during your second main phase, you won’t be able to cast another creature spell from your graveyard with Muldrotha’s permission until after your next ending phase ends.

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.