Creature — Elemental
As Nyxathid enters the battlefield, choose an opponent. Nyxathid gets -1/-1 for each card in the chosen player's hand.
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 the battlefield.
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.)