Do the Myriad tokens on say, a Herald of The Host, come out after you've declared the legal attack in Pramikon's chosen direction and bypass the attack restriction?
A creature entering the battlefield attacking a particular player or planeswalker, such as via myriad (C.R. 702.115a), can do so regardless of any "requirements or restrictions that apply to the declaration of attackers", including Pramikon's attacking restriction (C.R. 508.4c, 508.4).
EDIT (Aug. 24): One rule was renumbered with Commander 2019.
Myriad lets tokens enter the battlefield attacking opponents who might not be allowed to be defending players for you at all, perhaps because the Attack Multiple Players format option is not used. I'm confident it defeats any conceivable restriction that relates to getting into combat with a player. It definitely beats restrictions on the attack declaration, because the tokens don't attack.
I'm not convinced any existing rule explains what Pramikon is doing, since no multiplayer rule or standard one says what it means for a player to attack a player*. There's just this:
508.6. A player is “attacking [a player]” if the first player controls a creature that is attacking the second player. A player has “attacked [a player]” if the first player declared one or more creatures as attackers attacking the second player.
The second sentence seems like the expression of the intent, although it's oddly limited to the past tense. So Pramikon must mean "You can't declare anything to attack a player who's not the one in the chosen direction", assuming that the 'may' used in its already-unusual text is a red herring.
*Edit:
506.2a During the combat phase of a multiplayer game, there may be one or more defending players, depending on the variant being played and the options chosen for it. Unless all the attacking player’s opponents automatically become defending players during the combat phase, the attacking player chooses one of their opponents as a turn-based action during the beginning of combat step. (Note that the choice may be dictated by the variant being played or the options chosen for it.) That player becomes the defending player. See rule 802, “Attack Multiple Players Option,” rule 803, “Attack Left and Attack Right Options,” and rule 809, “Emperor Variant.”
EDIT (Aug. 24): One rule was renumbered with Commander 2019.
I'm not convinced any existing rule explains what Pramikon is doing, since no multiplayer rule or standard one says what it means for a player to attack a player*. There's just this:
508.6. A player is “attacking [a player]” if the first player controls a creature that is attacking the second player. A player has “attacked [a player]” if the first player declared one or more creatures as attackers attacking the second player.
The second sentence seems like the expression of the intent, although it's oddly limited to the past tense. So Pramikon must mean "You can't declare anything to attack a player who's not the one in the chosen direction", assuming that the 'may' used in its already-unusual text is a red herring.
*Edit:
506.2a During the combat phase of a multiplayer game, there may be one or more defending players, depending on the variant being played and the options chosen for it. Unless all the attacking player’s opponents automatically become defending players during the combat phase, the attacking player chooses one of their opponents as a turn-based action during the beginning of combat step. (Note that the choice may be dictated by the variant being played or the options chosen for it.) That player becomes the defending player. See rule 802, “Attack Multiple Players Option,” rule 803, “Attack Left and Attack Right Options,” and rule 809, “Emperor Variant.”
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