Nyxathid's two abilities are linked: The second one refers only to the player chosen by the first one. If another creature becomes a copy of Nyxathid (due to Mirrorweave, for example), the second ability won't do anything because a player was never chosen as a result of the first ability. This is true even if a different ability allowed a player to be chosen as that creature entered.
2009-02-01
wotc
Nyxathid tracks the hand size of the chosen player even if that player ceases to be an opponent of its controller. (This could happen if that player gains control of Nyxathid, for example.)
2009-02-01
wotc
In a multiplayer game, if the chosen player leaves the game, Nyxathid's second ability simply won't do anything anymore.
2009-02-01
wotc
If a spell or ability causes the chosen player to draw cards and then discard cards, Nyxathid's power and toughness changes accordingly as that spell or ability is resolving. For example, if the chosen player has five cards in hand and a spell causes that player to draw two cards then discard two cards, Nyxathid will start out as 2/2, then become 1/1, 0/0, and finally 2/2. This is because cards are always drawn one at a time, but multiple cards may be discarded at once. Although Nyxathid momentarily had 0 toughness while that spell was resolving, it isn't put into its owner's graveyard as a state-based action because state-based actions aren't checked until the spell has finished resolving.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Legal
Timeless
Legal
Gladiator
Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Legal
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