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Warchief Giant

Haste
Myriad (Whenever this creature attacks, for each opponent other than defending player, you may create a token that’s a copy of this creature that’s tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)

Rarity : Uncommon
Types : Creature
SubTypes : Giant, Warrior
Artist : Slawomir Maniak
Cost converted to mana : 5

Rulings

4 November 2015
The term “defending player” in the myriad rules (or any other ability of an attacking creature) refers to the player the creature with myriad was attacking at the time it became an attacking creature this combat, or the controller of the planeswalker the creature was attacking at the time it became an attacking creature this combat.
4 November 2015
If the defending player is your only opponent, no tokens are put onto the battlefield.
4 November 2015
You choose whether each token is attacking the player or a planeswalker they control as the token is created.
4 November 2015
Although the tokens enter the battlefield attacking, they were never declared as attackers. Abilities that trigger whenever a creature attacks won’t trigger, including the myriad ability of the tokens. If there any costs to have a creature attack, those costs won’t apply to the tokens.
4 November 2015
The token creatures all enter the battlefield at the same time.
4 November 2015
Each token copies exactly what was printed on the original creature and nothing else. It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
4 November 2015
Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “as [this permanent] enters the battlefield” or “[this permanent] enters the battlefield with” abilities of the copied creature will also work.
4 November 2015
If myriad creates more than one token for any given player (due to an effect such as the one Doubling Season creates), you may choose separately for each token whether it’s attacking the player or a planeswalker they control.

Arcane Sanctum

Arcane Sanctum enters the battlefield tapped. {T}: Add {W}, {U}, or {B}.

Sewer Nemesis

As Sewer Nemesis enters the battlefield, choose a player. Sewer Nemesis's power and toughness are each equal to the number of cards in the chosen player's graveyard. Whenever the chosen player casts a spell, that player puts the top card of their library into their graveyard.