"The next punch you throw hits the back of your own head."

Portal Manipulator

Flash
When this creature enters during the declare attackers step, choose target player and any number of target attacking creatures their opponents control. Those creatures are now attacking that player.

Ravnica: Clue Edition

Flash
When Portal Manipulator enters the battlefield during the declare attackers step, choose target player and any number of target attacking creatures their opponents control. Those creatures are now attacking that player.

If an ability targets something controlled by the "defending player" of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won't resolve because its target has become illegal.

Once Portal Manipulator's triggered ability resolves, the target attacking creatures are still considered to have attacked the player, planeswalker, or battle they were initially declared as attacking, but they are now attacking the target player.

Portal Manipulator's triggered ability doesn't cause "whenever this creature attacks" abilities to trigger.

Portal Manipulator's triggered ability ignores all requirements, restrictions, and costs associated with attacking.

You may cast Portal Manipulator outside of a declare attackers step. If Portal Manipulator enters the battlefield outside of a declare attackers step, its ability simply doesn't trigger.