This question is a bit complicated, so I'll go through it step-by-step.
Suppose that, in a game of Commander, I cast Scion of the Ur-Dragon, then use its activated ability to search my library for a dragon permanent card, put that card into my graveyard, and make Scion into a copy of that dragon until the end of my turn. (Let's say that the dragon permanent card is Mirrorwing Dragon.) Scion is now a copy of Mirrorwing Dragon, but Scion's copy ability is lost for the remainder of the turn.
What happens if I now equip the Scion-Mirrorwing creature with Blade of Selves, giving it myriad? ("Whenever it attacks, for each opponent other than defending player, you may create a token that's a copy of that creature that's tapped and attacking that player or a planeswalker they control.") Are these tokens copies of Mirrorwing Dragon, or are they (technically) copies of Scion of the Ur-Dragon? This is an important point because Scion is a legendary creature and Mirrowing Dragon isn't. Using Blade of Selves on Scion, of the Ur-Dragon would break the legend rule, but using it on Mirrorwing Dragon wouldn't.
I'm familiar with the rulings on Blade of Selves. Each token copies exactly what was printed on the original creature and nothing else, so it doesn't copy any counters, or any auras or equipment on the original creature. The token copies can't inflict commander damage, either. So, for example, I can't attach Grafted Exoskeleton to the Scion-Mirrorwing creature to give all of the token copies infect. But what if I cast Tainted Strike targeting the Scion-Mirrorwing creature? Will Mirrorwing Dragon's ability copy Tainted Strike for all of my creatures, including the token copies of the Scion-Mirrorwing creature?
A copy of a copy gets to look like the object that was first copied. Your Myriad tokens will be Mirrorwing Dragons. (rule 706.3)
Effects from Tainted Strike are not copied. (rule 706.2)
When you say that the effects from Tainted Strike are not copied (rule 706.2), do you mean that the Tainted Strike spell can't be copied by any of the Mirrorwing Dragon copies-- giving all of the creatures that I control infect until end of turn? Or do you mean that the multiple token copies made by Blade of Selves don't copy the Tainted Strike spell during their creation, granting them the ability to give all my creatures infect in and of themselves?
If, after using Blade of Selves to make multiple copies of Mirrorwing Dragon, I target any of the Mirrorwing Dragon copies using Tainted Strike, I would expect that ALL of my creatures would now have infect until end of turn, including the myriad-generated copies. Wouldn't the myriad-generated token copies of Mirrorwing Dragon also have infect now, since they are also creatures that I control?
Woops! I misread Mirrorwing Dragon! My bad.
Yes, Tainted Strike gets copied for each of your creature. Just make sure you use it AFTER the tokens get created.
A closely-related question... I assume that the original Mirrorwing Dragon copy made by Scion of the Ur-Dragon can inflict commander damage, as it is Scion of the Ur-Dragon made into a copy of Mirrorwing Dragon. However, none of the Blade of Selves token copies will inflict commander damage, as they are copies of the commander, not the commander.
But suppose that instead of using Blade of Selves, I have Saskia the Unyielding in play. Saskia asks me to choose a player as Saskia the Unyielding comes into play, and says that whenever a creature that I control deals combat damage to a player, it also deals that much damage to the chosen player. If the card that I'm attacking with is my commander, and I manage to inflict 10 commander damage to the first player, does the damage that Saskia deals to the second (chosen) player constitute combat damage, too? Or just regular damage? I would think that it ought to be commander damage, in this case, because the damage was inflicted by my commander through combat.
But suppose that instead of using Blade of Selves, I have Saskia the Unyielding in play. Saskia asks me to choose a player as Saskia the Unyielding comes into play, and says that whenever a creature that I control deals combat damage to a player, it also deals that much damage to the chosen player. If the card that I'm attacking with is my commander, and I manage to inflict 10 commander damage to the first player, does the damage that Saskia deals to the second (chosen) player constitute combat damage, too? Or just regular damage? I would think that it ought to be commander damage, in this case, because the damage was inflicted by my commander through combat.
Only combat damage dealt by commanders matters for the purposes of a state-based action added in the traditional Commander variant (C.R. 903.10a). Whatever damage is dealt when Saskia's triggered ability resolves is not combat damage, even if a commander deals that damage (compare C.R. 120.2b, which describes damage due to an effect, with C.R. 120.2a, which describes combat damage).
(Note that "it deals..." on Saskia's triggered ability refers to the creature that made that ability trigger, not necessarily Saskia itself.)
Suppose that, in a game of Commander, I cast Scion of the Ur-Dragon, then use its activated ability to search my library for a dragon permanent card, put that card into my graveyard, and make Scion into a copy of that dragon until the end of my turn. (Let's say that the dragon permanent card is Mirrorwing Dragon.) Scion is now a copy of Mirrorwing Dragon, but Scion's copy ability is lost for the remainder of the turn.
What happens if I now equip the Scion-Mirrorwing creature with Blade of Selves, giving it myriad? ("Whenever it attacks, for each opponent other than defending player, you may create a token that's a copy of that creature that's tapped and attacking that player or a planeswalker they control.") Are these tokens copies of Mirrorwing Dragon, or are they (technically) copies of Scion of the Ur-Dragon? This is an important point because Scion is a legendary creature and Mirrowing Dragon isn't. Using Blade of Selves on Scion, of the Ur-Dragon would break the legend rule, but using it on Mirrorwing Dragon wouldn't.
I'm familiar with the rulings on Blade of Selves. Each token copies exactly what was printed on the original creature and nothing else, so it doesn't copy any counters, or any auras or equipment on the original creature. The token copies can't inflict commander damage, either. So, for example, I can't attach Grafted Exoskeleton to the Scion-Mirrorwing creature to give all of the token copies infect. But what if I cast Tainted Strike targeting the Scion-Mirrorwing creature? Will Mirrorwing Dragon's ability copy Tainted Strike for all of my creatures, including the token copies of the Scion-Mirrorwing creature?
Effects from Tainted Strike are not copied. (rule 706.2)
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
If, after using Blade of Selves to make multiple copies of Mirrorwing Dragon, I target any of the Mirrorwing Dragon copies using Tainted Strike, I would expect that ALL of my creatures would now have infect until end of turn, including the myriad-generated copies. Wouldn't the myriad-generated token copies of Mirrorwing Dragon also have infect now, since they are also creatures that I control?
Yes, Tainted Strike gets copied for each of your creature. Just make sure you use it AFTER the tokens get created.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
A closely-related question... I assume that the original Mirrorwing Dragon copy made by Scion of the Ur-Dragon can inflict commander damage, as it is Scion of the Ur-Dragon made into a copy of Mirrorwing Dragon. However, none of the Blade of Selves token copies will inflict commander damage, as they are copies of the commander, not the commander.
But suppose that instead of using Blade of Selves, I have Saskia the Unyielding in play. Saskia asks me to choose a player as Saskia the Unyielding comes into play, and says that whenever a creature that I control deals combat damage to a player, it also deals that much damage to the chosen player. If the card that I'm attacking with is my commander, and I manage to inflict 10 commander damage to the first player, does the damage that Saskia deals to the second (chosen) player constitute combat damage, too? Or just regular damage? I would think that it ought to be commander damage, in this case, because the damage was inflicted by my commander through combat.
(Note that "it deals..." on Saskia's triggered ability refers to the creature that made that ability trigger, not necessarily Saskia itself.)
See also this thread and this thread.
EDIT (Nov. 11): Edited.
EDIT (Jan. 1, 2021): Edited to conform to rule changes in the meantime.
Thanks!