Ixhel, Scion of Atraxa

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2023-02-04
  Official Ruling
  You pay the costs for an exiled card if you cast it. You may pay any alternative costs the card has rather than the card's mana cost.
• WWizards of the Coast2023-02-04
  Official Ruling
  Damage dealt by a creature with toxic grants the same number of counters regardless of how much damage is dealt. Notably, if a replacement effect modifies the damage in some way (such as that of Gratuitous Violence), the number of counters given remains unchanged.
• WWizards of the Coast2023-02-04
  Official Ruling
  If you leave the game, the cards remain exiled face down indefinitely. No player may look at them.
• WWizards of the Coast2023-02-04
  Official Ruling
  Ixhel doesn't change when you can cast an exiled card. For example, if you exile a creature card without flash, you can cast it only during your main phase when the stack is empty.
• WWizards of the Coast2023-02-04
  Official Ruling
  Conversely, replacement effects that apply to the number of counters put on a player can modify the counters placed this way. For example, Vorinclex, Monstrous Raider's last two abilities can apply to counters placed this way.
• WWizards of the Coast2023-02-04
  Official Ruling
  You may look at and cast the exiled cards (and spend mana as though it were mana of any color to do so) even if Ixhel leaves the battlefield. If another player gains control of Ixhel, that player can't look at or cast the cards, and you still can.
• WWizards of the Coast2023-02-04
  Official Ruling
  If a creature with toxic deals combat damage to a creature or planeswalker, or if it deals noncombat damage, toxic has no effect and no player gets poison counters.
• WWizards of the Coast2023-02-04
  Official Ruling
  A player with ten or more poison counters loses the game. This is a state-based action and doesn't use the stack. In other words, it happens immediately and players can't respond to it, just like a player losing the game due to having 0 or less life.
• WWizards of the Coast2023-02-04
  Official Ruling
  Casting an exiled card causes it to leave exile. You can't cast it multiple times.
• WWizards of the Coast2023-02-04
  Official Ruling
  Any other effects of that damage, such as life gain from lifelink, still apply.
• WWizards of the Coast2023-02-04
  Official Ruling
  Toxic doesn't change the amount of combat damage a creature deals. For example, if a 2/2 creature with toxic 1 deals combat damage to a player, that creature will deal 2 damage. The results of that damage are the player loses 2 life and gets a poison counter.
• WWizards of the Coast2023-02-04
  Official Ruling
  Multiple instances of toxic are cumulative. For example, if a creature has toxic 2 and gains toxic 1 due to another effect, combat damage that creature deals to a player will cause that player to get 3 poison counters.

Rulings via Scryfall