Rielle, the Everwise

Rielle gets +1/+0 for each instant and sorcery card in your graveyard.
Whenever you discard one or more cards for the first time each turn, draw that many cards.

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Ikoria: Lair of Behemoths

If the first time you discard cards during your turn is due to having too many cards in hand during your cleanup step, you'll draw that many cards. Players will receive priority during the cleanup step in this case, and a new cleanup step will happen after that one, during which you may have to discard cards again.

If you discard a card as a cost to cast a spell or activate an ability, Rielle's triggered ability resolves before that spell or ability but after you've chosen targets for it. If you discard a card while a spell or ability is resolving, that spell or ability finishes resolving before Rielle's triggered ability does.'

Rielle's first ability applies only while it's on the battlefield.